diff --git a/README.md b/README.md
index fa8a2a0..fb79407 100644
--- a/README.md
+++ b/README.md
@@ -231,6 +231,12 @@ Montrose.every(:month, mday: [2, 15], total: 10)
 # monthly on the first and last day of the month for 10 occurrences
 Montrose.monthly(mday: [1, -1], total: 10)
 
+# monthly on the 30th unless fewer days
+Montrose.monthly(mday: { default: 30, fallback: -1 })
+
+# monthly on the 25th except in december
+Montrose.monthly(mday: { default: 25, december: 20 })
+
 # every 18 months on the 10th thru 15th of the month for 10 occurrences
 Montrose.every(18.months, total: 10, mday: 10..15)
 
diff --git a/lib/montrose/options.rb b/lib/montrose/options.rb
index 34a9736..fc717a3 100644
--- a/lib/montrose/options.rb
+++ b/lib/montrose/options.rb
@@ -206,8 +206,8 @@ def day=(days)
       @day = Day.parse(days)
     end
 
-    def mday=(mdays)
-      @mday = MonthDay.parse(mdays)
+    def mday=(mday_arg)
+      @mday = decompose_mday_arg(mday_arg)
     end
 
     def yday=(ydays)
@@ -370,5 +370,38 @@ def end_of_day
     def beginning_of_day
       @beginning_of_day ||= time_of_day_parse(Time.now.beginning_of_day)
     end
+
+    def decompose_mday_arg(mday_arg)
+      case mday_arg
+      when Hash
+        return nil unless mday_arg[:default].present?
+        {
+          default: MonthDay.parse(mday_arg[:default]),
+          overrides: flatten_mday_arg(mday_arg),
+          fallback: single_day(mday_arg[:fallback])
+        }
+      else
+        {default: MonthDay.parse(mday_arg), overrides: {}, fallback: nil}
+      end
+    end
+
+    def flatten_mday_arg(mday_arg)
+      mday_arg.except(:default, :overrides, :fallback).each_with_object({}) do |(months, day), result|
+        case months
+        when Array
+          months.each { |month| result[month] = single_day(day) }
+        else
+          result[months] = single_day(day)
+        end
+      end
+    end
+
+    def single_day(day_number)
+      return nil unless day_number
+      raise ConfigurationError, "mday override #{day_number} must be an integer" unless day_number.is_a?(Integer)
+      MonthDay.assert(day_number)
+
+      day_number
+    end
   end
 end
diff --git a/lib/montrose/rule/day_of_month.rb b/lib/montrose/rule/day_of_month.rb
index 01da054..3c6550c 100644
--- a/lib/montrose/rule/day_of_month.rb
+++ b/lib/montrose/rule/day_of_month.rb
@@ -11,22 +11,46 @@ def self.apply_options(opts)
 
       # Initializes rule
       #
-      # @param [Array<Fixnum>] days - valid days of month, i.e. [1, 2, -1]
+      # @param [Hash] opts `mday` valid days of month, and `skip_months` options
       #
-      def initialize(days)
-        @days = days
+      def initialize(opts)
+        @days = opts.fetch(:default)
+        @overrides = opts.fetch(:overrides, {})
+        @fallback = opts.fetch(:fallback, nil)
       end
 
       def include?(time)
-        @days.include?(time.mday) || included_from_end_of_month?(time)
+        return override?(time) if has_override?(month_name(time))
+
+        @days.include?(time.mday) || included_from_end_of_month?(time) || fallback?(time)
       end
 
       private
 
       # matches days specified at negative numbers
-      def included_from_end_of_month?(time)
+      def included_from_end_of_month?(time, days = @days)
         month_days = ::Montrose::Utils.days_in_month(time.month, time.year) # given by activesupport
-        @days.any? { |d| month_days + d + 1 == time.mday }
+        days.any? { |d| month_days + d + 1 == time.mday }
+      end
+
+      def has_override?(month)
+        @overrides.key?(month)
+      end
+
+      def override?(time)
+        return false if @overrides.blank?
+
+        time.day == @overrides[month_name(time)]
+      end
+
+      def month_name(time)
+        time.strftime("%B").downcase.to_sym
+      end
+
+      def fallback?(time)
+        return false unless @fallback.present?
+
+        time.day == @fallback || included_from_end_of_month?(time, [@fallback])
       end
     end
   end
diff --git a/spec/montrose/options_spec.rb b/spec/montrose/options_spec.rb
index 8770f98..e40d9a3 100644
--- a/spec/montrose/options_spec.rb
+++ b/spec/montrose/options_spec.rb
@@ -423,29 +423,57 @@
     it "can be set" do
       options[:mday] = [1, 20, 31]
 
-      _(options.mday).must_equal [1, 20, 31]
-      _(options[:mday]).must_equal [1, 20, 31]
+      _(options.mday).must_equal({default: [1, 20, 31], overrides: {}, fallback: nil})
+      _(options[:mday]).must_equal({default: [1, 20, 31], overrides: {}, fallback: nil})
+    end
+
+    it "can be set to a hash" do
+      options[:mday] = {default: [1, 20, 31]}
+
+      _(options.mday).must_equal({default: [1, 20, 31], overrides: {}, fallback: nil})
+      _(options[:mday]).must_equal({default: [1, 20, 31], overrides: {}, fallback: nil})
     end
 
     it "casts to element to array" do
       options[:mday] = 1
 
-      _(options.mday).must_equal [1]
-      _(options[:mday]).must_equal [1]
+      _(options.mday).must_equal({default: [1], overrides: {}, fallback: nil})
+      _(options[:mday]).must_equal({default: [1], overrides: {}, fallback: nil})
+    end
+
+    it "casts default element to array" do
+      options[:mday] = {default: 1}
+
+      _(options.mday).must_equal({default: [1], overrides: {}, fallback: nil})
+      _(options[:mday]).must_equal({default: [1], overrides: {}, fallback: nil})
     end
 
     it "allows negative numbers" do
-      options[:yday] = [-1]
+      options[:mday] = [-1]
 
-      _(options.yday).must_equal [-1]
-      _(options[:yday]).must_equal [-1]
+      _(options.mday).must_equal({default: [-1], overrides: {}, fallback: nil})
+      _(options[:mday]).must_equal({default: [-1], overrides: {}, fallback: nil})
+    end
+
+    it "allows default negative numbers" do
+      options[:mday] = {default: [-1]}
+
+      _(options.mday).must_equal({default: [-1], overrides: {}, fallback: nil})
+      _(options[:mday]).must_equal({default: [-1], overrides: {}, fallback: nil})
     end
 
     it "casts range to array" do
       options[:mday] = 6..8
 
-      _(options.mday).must_equal [6, 7, 8]
-      _(options[:mday]).must_equal [6, 7, 8]
+      _(options.mday).must_equal({default: [6, 7, 8], overrides: {}, fallback: nil})
+      _(options[:mday]).must_equal({default: [6, 7, 8], overrides: {}, fallback: nil})
+    end
+
+    it "casts default range to array" do
+      options[:mday] = {default: 6..8}
+
+      _(options.mday).must_equal({default: [6, 7, 8], overrides: {}, fallback: nil})
+      _(options[:mday]).must_equal({default: [6, 7, 8], overrides: {}, fallback: nil})
     end
 
     it "casts nil to empty array" do
@@ -455,9 +483,64 @@
       _(options[:day]).must_be_nil
     end
 
+    it "casts default nil to empty array" do
+      options[:mday] = {default: nil}
+
+      _(options.mday).must_be_nil
+      _(options[:mday]).must_be_nil
+    end
+
     it "raises for out of range" do
       _(-> { options[:mday] = [1, 100] }).must_raise
     end
+
+    it "raises for default out of range" do
+      _(-> { options[:mday] = {default: [1, 100]} }).must_raise
+    end
+
+    it "raises for array override" do
+      _(-> { options[:mday] = {default: 31, february: [28, 29]} }).must_raise
+    end
+
+    it "raises for array fallback" do
+      _(-> { options[:mday] = {default: 31, fallback: [28, 29]} }).must_raise
+    end
+
+    it "allows negative overrides" do
+      options[:mday] = {default: 31, february: -1}
+
+      _(options.mday).must_equal({default: [31], overrides: {february: -1}, fallback: nil})
+      _(options[:mday]).must_equal({default: [31], overrides: {february: -1}, fallback: nil})
+    end
+
+    it "allows negative fallback" do
+      options[:mday] = {default: 31, fallback: -1}
+
+      _(options.mday).must_equal({default: [31], overrides: {}, fallback: -1})
+      _(options[:mday]).must_equal({default: [31], overrides: {}, fallback: -1})
+    end
+
+    it "raises for override out of range" do
+      _(-> { options[:mday] = {default: 31, february: 100} }).must_raise
+    end
+
+    it "raises for fallback out of range" do
+      _(-> { options[:mday] = {default: 31, fallback: 100} }).must_raise
+    end
+
+    it "collects overrides" do
+      options[:mday] = {default: 31, september: 30, february: 28}
+
+      _(options.mday).must_equal({default: [31], overrides: {september: 30, february: 28}, fallback: nil})
+      _(options[:mday]).must_equal({default: [31], overrides: {september: 30, february: 28}, fallback: nil})
+    end
+
+    it "flattens overrides" do
+      options[:mday] = {:default => 31, [:september, :april, :june, :november] => 30, :february => 28}
+
+      _(options.mday).must_equal({default: [31], overrides: {september: 30, april: 30, june: 30, november: 30, february: 28}, fallback: nil})
+      _(options[:mday]).must_equal({default: [31], overrides: {september: 30, april: 30, june: 30, november: 30, february: 28}, fallback: nil})
+    end
   end
 
   describe "#yday" do
@@ -767,7 +850,7 @@
       options[:on] = {friday: 13}
 
       _(options[:day]).must_equal [5]
-      _(options[:mday]).must_equal [13]
+      _(options[:mday]).must_equal({default: [13], overrides: {}, fallback: nil})
       _(options[:on]).must_equal(friday: 13)
     end
 
@@ -776,7 +859,7 @@
       options[:on] = {tuesday: 2..8}
 
       _(options[:day]).must_equal [2]
-      _(options[:mday]).must_equal((2..8).to_a)
+      _(options[:mday]).must_equal({default: (2..8).to_a, overrides: {}, fallback: nil})
       _(options[:month]).must_equal [11]
     end
 
@@ -784,7 +867,7 @@
       options[:on] = {january: 31}
 
       _(options[:month]).must_equal [1]
-      _(options[:mday]).must_equal [31]
+      _(options[:mday]).must_equal({default: [31], overrides: {}, fallback: nil})
     end
 
     it { _(-> { options[:on] = -3 }).must_raise Montrose::ConfigurationError }
diff --git a/spec/montrose/rule/day_of_month_spec.rb b/spec/montrose/rule/day_of_month_spec.rb
index ff24faa..16c1457 100644
--- a/spec/montrose/rule/day_of_month_spec.rb
+++ b/spec/montrose/rule/day_of_month_spec.rb
@@ -3,7 +3,9 @@
 require "spec_helper"
 
 describe Montrose::Rule::DayOfMonth do
-  let(:rule) { Montrose::Rule::DayOfMonth.new([1, 10, -1]) }
+  let(:rule) { Montrose::Rule::DayOfMonth.new(default: [1, 10, -1], overrides: {}, fallback: nil) }
+  let(:fallback_rule) { Montrose::Rule::DayOfMonth.new(default: [31], overrides: {}, fallback: -1) }
+  let(:override_rule) { Montrose::Rule::DayOfMonth.new(default: [15], overrides: {february: 28, september: 30, november: 30, april: 30}, fallback: nil) }
 
   describe "#include?" do
     it { assert rule.include?(Time.local(2016, 1, 1)) }
@@ -14,6 +16,22 @@
     it { refute rule.include?(Time.local(2015, 1, 2)) }
     it { refute rule.include?(Time.local(2015, 1, 30)) }
     it { refute rule.include?(Time.local(2015, 2, 27)) }
+
+    it { assert fallback_rule.include?(Time.local(2016, 1, 31)) }
+    it { assert fallback_rule.include?(Time.local(2015, 2, 28)) }
+    it { assert fallback_rule.include?(Time.local(2016, 2, 29)) }
+    it { assert fallback_rule.include?(Time.local(2016, 4, 30)) }
+    it { refute fallback_rule.include?(Time.local(2016, 1, 30)) }
+
+    it { assert override_rule.include?(Time.local(2016, 1, 15)) }
+    it { assert override_rule.include?(Time.local(2016, 2, 28)) }
+    it { assert override_rule.include?(Time.local(2016, 9, 30)) }
+    it { assert override_rule.include?(Time.local(2016, 11, 30)) }
+    it { assert override_rule.include?(Time.local(2016, 4, 30)) }
+    it { refute override_rule.include?(Time.local(2016, 9, 15)) }
+    it { refute override_rule.include?(Time.local(2016, 11, 15)) }
+    it { refute override_rule.include?(Time.local(2016, 4, 15)) }
+    it { refute override_rule.include?(Time.local(2016, 2, 15)) }
   end
 
   describe "#continue?" do