diff --git a/Makefile b/Makefile
index adf7d14..1e4334d 100644
--- a/Makefile
+++ b/Makefile
@@ -1,9 +1,9 @@
-default: validate
+default: test
 
 validate:
 	bundle exec ruby lib/validation/run.rb
 
-test:
+test: validate
 	bundle exec rspec
 
 .PHONY: validate,test
diff --git a/au.yaml b/au.yaml
index 0bfb458..929a6d2 100644
--- a/au.yaml
+++ b/au.yaml
@@ -102,12 +102,33 @@ months:
   - name: ANZAC Day # ANZAC DAY ACTUAL
     regions: [au_nsw, au_sa, au_tas, au_vic, au_act, au_wa]
     mday: 25
+    year_ranges:
+      - before: 2025
   - name: ANZAC Day # ANZAC DAY OBSERVED - no additional recognised
     regions: [au_nt, au_qld]
     mday: 25
     observed: to_monday_if_weekend(date)
+    year_ranges:
+      - before: 2025
   - name: Additional public holiday for ANZAC Day # ADDITIONAL ANZAC DAY
-    regions: [au_sa, au_act, au_wa]
+    regions: [au_sa, au_act]
+    mday: 25
+    function: additional_anzac_on_monday_if_on_weekend(date)
+    year_ranges:
+      - before: 2025
+  - name: ANZAC Day # ANZAC DAY ACTUAL
+    regions: [au_nsw, au_sa, au_tas, au_vic, au_wa, au_nt]
+    mday: 25
+    year_ranges:
+      - after: 2026
+  - name: ANZAC Day # ANZAC DAY ACTUAL
+    regions: [au_qld, au_act]
+    mday: 25
+    function: to_monday_if_sunday(date)
+    year_ranges:
+      - after: 2026
+  - name: Additional public holiday for ANZAC Day # ADDITIONAL ANZAC DAY
+    regions: [au_wa]
     mday: 25
     function: additional_anzac_on_monday_if_on_weekend(date)
   5:
@@ -457,20 +478,55 @@ methods:
 
 tests:
   - given:
-      date: '2020-04-25'
-      regions: ["au_act"]
+      date: '2015-04-27'
+      regions: ["au_qld"]
+      options: ["observed"]
+    expect:
+      name: "ANZAC Day"
+  - given:
+      date: ['2020-04-25', '2021-04-25', '2022-04-25', '2023-04-25', '2024-04-25', '2025-04-25']
+      regions: ["au_nsw", "au_act", "au_vic", "au_tas", "au_sa", "au_wa"]
     expect:
       name: 'ANZAC Day'
   - given:
-      date: '2020-04-25'
-      regions: ["au_qld"]
+      date: ['2020-04-27', '2021-04-26', '2022-04-25', '2023-04-25', '2024-04-25', '2025-04-25']
+      regions: ["au_qld", "au_nt"]
+      options: ["observed"]
+    expect:
+      name: 'ANZAC Day'
+  - given:
+      date: ['2026-04-25', '2027-04-25']
+      regions: ["au_nsw", "au_vic", "au_tas", "au_sa", "au_wa", "au_nt"]
     expect:
       name: 'ANZAC Day'
   - given:
-      date: '2021-04-27'
+      # moves to Monday if falls on Sunday
+      date: ['2026-04-25', '2027-04-26', '2028-04-25', '2029-04-25', '2030-04-25', '2031-04-25', '2032-04-26', '2033-04-25', '2034-04-25', '2035-04-25']
+      regions: ["au_qld", "au_act"]
+    expect:
+      name: 'ANZAC Day'
+  - given:
+      # additional on Monday if falls on Weekend
+      date: ['2021-04-26', '2026-04-27', '2027-04-26']
       regions: ["au_wa"]
     expect:
       name: 'Additional public holiday for ANZAC Day'
+  - given:
+      date: '2015-04-25'
+      regions: ["au_wa"]
+    expect:
+      name: 'ANZAC Day'
+  - given:
+      date: '2015-04-27'
+      regions: ["au_wa"]
+      options: ["observed"]
+    expect:
+      name: 'Additional public holiday for ANZAC Day'
+  - given:
+      date: '2026-04-27'
+      regions: ["au_nsw", "au_act", "au_vic", "au_tas", "au_sa", "au_nt", "au_qld"]
+    expect:
+      holiday: false
   - given:
       date: '2021-12-25'
       regions: ["au_qld"]
@@ -490,7 +546,7 @@ tests:
       date: '2021-12-28'
       regions: ["au_qld"]
     expect:
-      name: 'Additional public holiday for Boxing Day'
+      name: 'Additional public holiday Boxing Day'
   - given:
       date: '2013-10-07'
       regions: ["au_qld"]
@@ -527,7 +583,7 @@ tests:
     expect:
       name: "Queen's Birthday"
   - given:
-      date: ['2023-09-25','2024-09-23', '2025-09-30']
+      date: ['2023-09-25','2024-09-23', '2025-09-29']
       regions: ["au_wa"]
     expect:
       name: "King's Birthday"
@@ -554,12 +610,12 @@ tests:
       name: 'ACT Reconciliation Day'
   - given:
       date: '2014-01-26'
-      regions: ["au_qld"]
+      regions: ["au"]
     expect:
       name: 'Australia Day'
   - given:
       date: '2014-01-27'
-      regions: ["au_qld"]
+      regions: ["au"]
       options: ["observed"]
     expect:
       name: 'Australia Day'
@@ -613,21 +669,11 @@ tests:
       regions: ["au_tas_north"]
     expect:
       name: 'Recreation Day'
-  - given:
-      date: '2015-12-26'
-      regions: ["au_tas"]
-    expect:
-      holiday: false
   - given:
       date: '2015-11-14'
       regions: ["au_qld_brisbane"]
     expect:
       holiday: false
-  - given:
-      date: '2015-12-26'
-      regions: ["au_nt"]
-    expect:
-      holiday: false
   - given:
       date: '2016-12-27'
       regions: ["au_sa"]
@@ -638,16 +684,6 @@ tests:
       regions: ["au_nt"]
     expect:
       name: 'Christmas Day'
-  - given:
-      date: '2014-11-04'
-      regions: ["au_vic"]
-    expect:
-      holiday: false
-  - given:
-      date: '2015-11-03'
-      regions: ["au_vic"]
-    expect:
-      holiday: false
   - given:
       date: '2016-12-25'
       regions: ["au_vic"]
@@ -663,17 +699,6 @@ tests:
       regions: ["au_sa"]
     expect:
       holiday: false
-  - given:
-      date: '2015-12-26'
-      regions: ["au_sa"]
-    expect:
-      holiday: false
-  - given:
-      date: '2015-04-27'
-      regions: ["au_qld"]
-      options: ["observed"]
-    expect:
-      holiday: false
   - given:
       date: '2014-11-14'
       regions: ["au_qld"]
@@ -681,17 +706,17 @@ tests:
       holiday: false
   - given:
       date: '2014-11-04'
-      regions: ["au_vic_melbourne"]
+      regions: ["au_vic_melbourne", "au_vic"]
     expect:
       name: 'Melbourne Cup Day'
   - given:
       date: '2015-11-03'
-      regions: ["au_vic_melbourne"]
+      regions: ["au_vic_melbourne", "au_vic"]
     expect:
       name: 'Melbourne Cup Day'
   - given:
       date: '2019-11-05'
-      regions: ["au_vic"]
+      regions: ["au_vic", "au_vic_melbourne"]
     expect:
       name: 'Melbourne Cup Day'
   - given:
@@ -774,17 +799,6 @@ tests:
       regions: ["au_sa"]
     expect:
       name: 'March Public Holiday'
-  - given:
-      date: '2015-04-25'
-      regions: ["au_qld", "au_wa"]
-    expect:
-      name: 'ANZAC Day'
-  - given:
-      date: '2015-04-27'
-      regions: ["au_wa"]
-      options: ["observed"]
-    expect:
-      name: 'ANZAC Day'
   - given:
       date: '2015-12-26'
       regions: ["au_qld"]
@@ -795,17 +809,17 @@ tests:
       regions: ["au_qld"]
       options: ["observed"]
     expect:
-      name: "Boxing Day"
+      name: "Additional public holiday Boxing Day"
   - given:
       date: '2020-12-26'
       regions: ["au_qld"]
     expect:
-      name: "Additional public holiday for Boxing Day"
+      name: "Boxing Day"
   - given:
       date: '2015-12-26'
       regions: ["au_sa"]
     expect:
-      holiday: false
+      name: "Boxing Day"
   - given:
       date: '2015-12-28'
       regions: ["au_sa"]
@@ -814,23 +828,30 @@ tests:
   - given:
       date: '2015-12-26'
       regions: ["au_tas"]
+    expect:
+      name: "Boxing Day"
+  - given:
+      date: '2015-12-26'
+      regions: ["au_tas"]
+      options: ["observed"]
     expect:
       holiday: false
   - given:
       date: '2015-12-28'
       regions: ["au_tas"]
+      options: ["observed"]
     expect:
       name: "Boxing Day"
   - given:
       date: '2015-12-26'
       regions: ["au_nt"]
     expect:
-      holiday: false
+      name: "Boxing Day"
   - given:
       date: '2015-12-28'
       regions: ["au_nt"]
     expect:
-      name: "Boxing Day"
+      name: "Additional public holiday Boxing Day"
   - given:
       date: '2012-02-13'
       regions: ["au_tas_south"]
@@ -860,9 +881,8 @@ tests:
   - given:
       date: '2016-12-27'
       regions: ["au_qld"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-25'
       regions: ["au_nsw"]
@@ -871,9 +891,8 @@ tests:
   - given:
       date: '2016-12-27'
       regions: ["au_nsw"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-25'
       regions: ["au_act"]
@@ -882,9 +901,8 @@ tests:
   - given:
       date: '2016-12-27'
       regions: ["au_act"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-25'
       regions: ["au_tas"]
@@ -893,9 +911,8 @@ tests:
   - given:
       date: '2016-12-27'
       regions: ["au_tas"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-25'
       regions: ["au_wa"]
@@ -904,9 +921,8 @@ tests:
   - given:
       date: '2016-12-27'
       regions: ["au_wa"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-25'
       regions: ["au_sa"]
@@ -927,7 +943,7 @@ tests:
       regions: ["au_sa"]
       options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-25'
       regions: ["au_nsw"]
@@ -936,9 +952,8 @@ tests:
   - given:
       date: '2016-12-27'
       regions: ["au_nsw"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-25'
       regions: ["au_act"]
@@ -947,9 +962,8 @@ tests:
   - given:
       date: '2016-12-27'
       regions: ["au_act"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-25'
       regions: ["au_tas"]
@@ -958,9 +972,8 @@ tests:
   - given:
       date: '2016-12-27'
       regions: ["au_tas"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-25'
       regions: ["au_wa"]
@@ -969,9 +982,8 @@ tests:
   - given:
       date: '2016-12-27'
       regions: ["au_wa"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-25'
       regions: ["au_sa"]
@@ -980,42 +992,28 @@ tests:
   - given:
       date: '2016-12-26'
       regions: ["au_sa"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2016-12-27'
       regions: ["au_vic"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
-      date: '2016-12-27'
+      date: '2016-12-26'
       regions: ["au_nt"]
-      options: ["observed"]
     expect:
-      name: 'Christmas Day'
+      name: 'Additional public holiday for Christmas Day'
   - given:
       date: '2017-01-01'
-      regions: ["au", "au_qld", "au_nsw", "au_act", "au_vic", "au_sa", "au_wa", "au_nt"]
+      regions: ["au_qld", "au_nsw", "au_act", "au_vic", "au_wa", "au_nt"]
     expect:
       name: "New Year's Day"
-
-#FIXME This commented test is valid but the current code in the ruby repo won't allow it
-#      to work. We need to discuss potential solutions but in the meantime I'm taking it out.
-#      See https://github.com/holidays/definitions/issues/37.
-#  - given:
-#      date: '2017-01-01'
-#      regions: ["au_tas"]
-#    expect:
-#      holiday: false
-
   - given:
       date: '2017-01-02'
-      regions: ["au", "au_qld", "au_nsw", "au_act", "au_vic", "au_sa", "au_wa", "au_nt"]
-      options: ["observed"]
+      regions: ["au_qld", "au_nsw", "au_act", "au_vic", "au_sa", "au_wa", "au_nt"]
     expect:
-      name: "New Year's Day"
+      name: "Additional public holiday for New Year's Day"
   - given:
       date: '2017-01-02'
       regions: ["au_tas"]
@@ -1023,12 +1021,12 @@ tests:
       name: "New Year's Day"
   - given:
       date: '2022-01-03'
-      regions: ["au_nsw, au_vic, au_act, au_sa, au_wa, au_nt, au_qld"]
+      regions: ["au_nsw", "au_vic", "au_act", "au_sa", "au_wa", "au_nt", "au_qld"]
     expect:
       name: "Additional public holiday for New Year's Day"
   - given:
       date: '2022-01-01'
-      regions: ["au_nsw, au_vic, au_act, au_sa, au_wa, au_nt, au_qld"]
+      regions: ["au_nsw", "au_vic", "au_act", "au_wa", "au_nt", "au_qld"]
     expect:
       name: "New Year's Day"
   - given:
@@ -1048,14 +1046,9 @@ tests:
       name: "Easter Sunday"
   - given:
       date: '2021-08-02'
-      regions: ["au-nt"]
+      regions: ["au_nt"]
     expect:
       name: "Picnic Day"
-  - given:
-      date: '2022-04-17'
-      regions: ["au-nt"]
-    expect:
-      holiday: false
   - given:
       date: '2023-04-09'
       regions: ["au_nt"]
diff --git a/lib/validation/error.rb b/lib/validation/error.rb
index d3095be..a8cb117 100644
--- a/lib/validation/error.rb
+++ b/lib/validation/error.rb
@@ -5,6 +5,7 @@ class NoMonths < Error ; end
     class InvalidMonth < Error; end
     class InvalidMethod < Error; end
     class InvalidRegions < Error; end
+    class MissingDateSpecification < Error; end
     class InvalidCustomMethod < Error; end
     class InvalidTest < Error; end
   end
diff --git a/lib/validation/month_validator.rb b/lib/validation/month_validator.rb
index 505751a..86cabc8 100644
--- a/lib/validation/month_validator.rb
+++ b/lib/validation/month_validator.rb
@@ -16,11 +16,41 @@ def call(months)
             month_def['regions'].each do |region|
               raise Errors::InvalidRegions.new("A month must contain at least one region, received: #{months}") if region.nil? || region.empty?
             end
+
+            # Validate that each holiday has at least one date specification
+            validate_date_specification!(month_def)
           end
         end
 
         true
       end
+
+      private
+
+      def validate_date_specification!(month_def)
+        has_mday = month_def.key?('mday') && !month_def['mday'].nil?
+        has_wday = month_def.key?('wday') && !month_def['wday'].nil?
+        has_week = month_def.key?('week') && !month_def['week'].nil?
+        has_function = month_def.key?('function') && !month_def['function'].nil? && !month_def['function'].empty?
+
+        holiday_name = month_def['name'] || 'Unknown Holiday'
+
+        # If wday is specified, week must also be specified (and vice versa)
+        if (has_wday && !has_week) || (!has_wday && has_week)
+          raise Errors::MissingDateSpecification.new(
+            "Holiday '#{holiday_name}' with 'wday' must also have 'week' attribute (and vice versa)"
+          )
+        end
+
+        # Must have either mday, function, or both wday AND week
+        valid_date_spec = has_mday || has_function || (has_wday && has_week)
+
+        unless valid_date_spec
+          raise Errors::MissingDateSpecification.new(
+            "Holiday '#{holiday_name}' must have either 'mday', 'function', or both 'wday' and 'week' attributes to specify the date"
+          )
+        end
+      end
     end
   end
 end
diff --git a/spec/validation/month_validator_spec.rb b/spec/validation/month_validator_spec.rb
index 53ff06c..ece85c0 100644
--- a/spec/validation/month_validator_spec.rb
+++ b/spec/validation/month_validator_spec.rb
@@ -68,4 +68,56 @@
       }
     end
   end
+
+  context 'date specification validation' do
+    it 'allows holiday with mday' do
+      months = { 1 => [{"name"=>"Test Holiday", "regions"=>["test"], "mday"=>1}] }
+      expect(subject.call(months)).to be true
+    end
+
+    it 'allows holiday with function' do
+      months = { 1 => [{"name"=>"Test Holiday", "regions"=>["test"], "function"=>"custom_method(year)"}] }
+      expect(subject.call(months)).to be true
+    end
+
+    it 'allows holiday with wday and week' do
+      months = { 1 => [{"name"=>"Test Holiday", "regions"=>["test"], "wday"=>1, "week"=>2}] }
+      expect(subject.call(months)).to be true
+    end
+
+    it 'returns error if holiday has no date specification' do
+      months = { 1 => [{"name"=>"Test Holiday", "regions"=>["test"]}] }
+      expect { subject.call(months) }.to raise_error(Definitions::Errors::MissingDateSpecification) { |e|
+        expect(e.message).to eq("Holiday 'Test Holiday' must have either 'mday', 'function', or both 'wday' and 'week' attributes to specify the date")
+      }
+    end
+
+    it 'returns error if holiday has wday but no week' do
+      months = { 1 => [{"name"=>"Test Holiday", "regions"=>["test"], "wday"=>1}] }
+      expect { subject.call(months) }.to raise_error(Definitions::Errors::MissingDateSpecification) { |e|
+        expect(e.message).to eq("Holiday 'Test Holiday' with 'wday' must also have 'week' attribute (and vice versa)")
+      }
+    end
+
+    it 'returns error if holiday has week but no wday' do
+      months = { 1 => [{"name"=>"Test Holiday", "regions"=>["test"], "week"=>1}] }
+      expect { subject.call(months) }.to raise_error(Definitions::Errors::MissingDateSpecification) { |e|
+        expect(e.message).to eq("Holiday 'Test Holiday' with 'wday' must also have 'week' attribute (and vice versa)")
+      }
+    end
+
+    it 'returns error if holiday has empty function' do
+      months = { 1 => [{"name"=>"Test Holiday", "regions"=>["test"], "function"=>""}] }
+      expect { subject.call(months) }.to raise_error(Definitions::Errors::MissingDateSpecification) { |e|
+        expect(e.message).to eq("Holiday 'Test Holiday' must have either 'mday', 'function', or both 'wday' and 'week' attributes to specify the date")
+      }
+    end
+
+    it 'returns error if holiday has nil function' do
+      months = { 1 => [{"name"=>"Test Holiday", "regions"=>["test"], "function"=>nil}] }
+      expect { subject.call(months) }.to raise_error(Definitions::Errors::MissingDateSpecification) { |e|
+        expect(e.message).to eq("Holiday 'Test Holiday' must have either 'mday', 'function', or both 'wday' and 'week' attributes to specify the date")
+      }
+    end
+  end
 end