diff --git a/README.md b/README.md
index 123953a..201e9e6 100644
--- a/README.md
+++ b/README.md
@@ -35,6 +35,7 @@ r = Recurrence.weekly(on: :thursday)
 # Monthly by month day
 r = Recurrence.new(every: :month, on: 15)
 r = Recurrence.new(every: :month, on: 31)
+r = Recurrence.new(:every => :month, :on => [15, 31])
 r = Recurrence.new(every: :month, on: 7, interval: 2)
 r = Recurrence.new(every: :month, on: 7, interval: :monthly)
 r = Recurrence.new(every: :month, on: 7, interval: :bimonthly)
@@ -79,6 +80,7 @@ r = Recurrence.new(every: :day, except: [Date.current, '2010-01-31'])
 r = Recurrence.new(every: :month, on: 1, handler: Proc.new { |day, month, year| raise("Date not allowed!") if year == 2011 && month == 12 && day == 31 })
 
 # Shift the recurrences to maintain dates around boundaries (Jan 31 -> Feb 28 -> Mar 28)
+# Shift cannot be used with multiple month days e.g: :on => [1,15]
 r = Recurrence.new(every: :month, on: 31, shift: true)
 
 # Getting an array with all events
diff --git a/lib/recurrence/event.rb b/lib/recurrence/event.rb
index b5c831e..7284871 100644
--- a/lib/recurrence/event.rb
+++ b/lib/recurrence/event.rb
@@ -5,6 +5,8 @@ module Event # :nodoc: all
     require "recurrence/event/base"
     require "recurrence/event/daily"
     require "recurrence/event/monthly"
+    require "recurrence/event/monthly/monthday"
+    require "recurrence/event/monthly/weekday"
     require "recurrence/event/weekly"
     require "recurrence/event/yearly"
   end
diff --git a/lib/recurrence/event/monthly.rb b/lib/recurrence/event/monthly.rb
index 63695f5..6c86310 100644
--- a/lib/recurrence/event/monthly.rb
+++ b/lib/recurrence/event/monthly.rb
@@ -12,27 +12,12 @@ class Monthly < Base # :nodoc: all
 
       private def validate
         if @options.key?(:weekday)
-
-          # Allow :on => :last, :weekday => :thursday contruction.
-          if @options[:on].to_s == "last"
-            @options[:on] = 5
-          elsif @options[:on].is_a?(Numeric)
-            valid_week?(@options[:on])
-          else
-            valid_ordinal?(@options[:on])
-            @options[:on] = ORDINALS.index(@options[:on].to_s) + 1
-          end
-
-          @options[:weekday] =
-            valid_weekday_or_weekday_name?(@options[:weekday])
+          extend Weekday
         else
-          valid_month_day?(@options[:on])
+          extend Monthday
         end
 
-        return unless @options[:interval].is_a?(Symbol)
-
-        valid_interval?(@options[:interval])
-        @options[:interval] = INTERVALS[@options[:interval]]
+        validate_and_prepare!
       end
 
       private def next_in_recurrence
@@ -100,8 +85,6 @@ class Monthly < Base # :nodoc: all
           weeks = (date.day - 1).div(7) + 1
           date -= weeks * 7
         end
-
-        @options[:handler].call(date.day, date.month, date.year)
       end
 
       private def shift_to(date)
diff --git a/lib/recurrence/event/monthly/monthday.rb b/lib/recurrence/event/monthly/monthday.rb
new file mode 100644
index 0000000..97229d0
--- /dev/null
+++ b/lib/recurrence/event/monthly/monthday.rb
@@ -0,0 +1,59 @@
+class Recurrence_
+  module Event
+    class Monthly < Base
+      module Monthday
+        def advance(date, interval=@options[:interval])
+          if initialized? && @_day_count > @_day_pointer += 1
+            @options[:handler].call(
+              @options[:on][@_day_pointer],
+              date.month,
+              date.year
+            )
+          else
+            @_day_pointer = 0
+
+            # Have a raw month from 0 to 11 interval
+            raw_month  = date.month + interval - 1
+
+            next_year  = date.year + raw_month.div(12)
+            next_month = (raw_month % 12) + 1 # change back to ruby interval
+
+            @options[:handler].call(
+              @options[:on][@_day_pointer],
+              next_month,
+              next_year
+            )
+          end
+        end
+
+        def validate_and_prepare!
+          @options[:on] = Array.wrap(@options[:on]).map do |day|
+            valid_month_day?(day)
+            day
+          end.sort
+
+          valid_shift_options?
+
+          if @options[:interval].kind_of?(Symbol)
+            valid_interval?(@options[:interval])
+            @options[:interval] = INTERVALS[@options[:interval]]
+          end
+
+          @_day_pointer = 0
+          @_day_count = @options[:on].length
+        end
+
+        def valid_shift_options?
+          if @options[:shift] && @options[:on].length > 1
+            raise ArgumentError, "Invalid options. Unable to use :shift with multiple :on days"
+          end
+        end
+
+        def shift_to(date)
+          @options[:on][0] = date.day
+        end
+      end
+    end
+  end
+end
+
diff --git a/lib/recurrence/event/monthly/weekday.rb b/lib/recurrence/event/monthly/weekday.rb
new file mode 100644
index 0000000..3662140
--- /dev/null
+++ b/lib/recurrence/event/monthly/weekday.rb
@@ -0,0 +1,53 @@
+class Recurrence_
+  module Event
+    class Monthly < Base
+      module Weekday
+        def advance(date, interval=@options[:interval])
+          raw_month  = date.month + interval - 1
+          next_year  = date.year + raw_month.div(12)
+          next_month = (raw_month % 12) + 1 # change back to ruby interval
+          date       = Date.new(next_year, next_month, 1)
+
+          weekday, month = @options[:weekday], date.month
+
+          # Adjust week day
+          to_add  = weekday - date.wday
+          to_add += 7 if to_add < 0
+          to_add += (@options[:on] - 1) * 7
+          date   += to_add
+
+          # Go to the previous month if we lost it
+          if date.month != month
+            weeks = (date.day - 1).div(7) + 1
+            date -= weeks * 7
+          end
+
+          @options[:handler].call(date.day, date.month, date.year)
+        end
+
+        def validate_and_prepare!
+          # Allow :on => :last, :weekday => :thursday contruction.
+          if @options[:on].to_s == "last"
+            @options[:on] = 5
+          elsif @options[:on].kind_of?(Numeric)
+            valid_week?(@options[:on])
+          else
+            valid_ordinal?(@options[:on])
+            @options[:on] = Monthly::ORDINALS.index(@options[:on].to_s) + 1
+          end
+
+          @options[:weekday] = valid_weekday_or_weekday_name?(@options[:weekday])
+
+          if @options[:interval].kind_of?(Symbol)
+            valid_interval?(@options[:interval])
+            @options[:interval] = INTERVALS[@options[:interval]]
+          end
+        end
+
+        def shift_to(date)
+          @options[:on] = date.day
+        end
+      end
+    end
+  end
+end
diff --git a/lib/recurrence/namespace.rb b/lib/recurrence/namespace.rb
index 463d81e..65234ab 100644
--- a/lib/recurrence/namespace.rb
+++ b/lib/recurrence/namespace.rb
@@ -100,6 +100,7 @@ def self.weekly(options = {})
   # Create a monthly recurrence.
   #
   #   Recurrence.monthly(on: 15) #=> every 15th day
+  #   Recurrence.monthly(:on => [1, 15]) #=> every 1st and 15th day
   #   Recurrence.monthly(on: :first, :weekday => :sunday)
   #   Recurrence.monthly(on: :second, :weekday => :sunday)
   #   Recurrence.monthly(on: :third, :weekday => :sunday)
diff --git a/test/recurrence/monthly_recurring/day_test.rb b/test/recurrence/monthly_recurring/day_test.rb
index c941dc3..e2ee841 100644
--- a/test/recurrence/monthly_recurring/day_test.rb
+++ b/test/recurrence/monthly_recurring/day_test.rb
@@ -144,4 +144,30 @@ class MonthlyRecurringDayTest < Minitest::Test
     assert r.events.include?(7.months.from_now.to_date)
     refute r.events.include?(8.months.from_now.to_date)
   end
+
+  test "allows multiple days" do
+    r = recurrence(
+      every: :month,
+      on: [1, 15],
+      starts: "2017-05-01",
+      until: "2017-06-30"
+    )
+    assert_equal "2017-05-01", r.events[0].to_s
+    assert_equal "2017-05-15", r.events[1].to_s
+    assert_equal "2017-06-01", r.events[2].to_s
+    assert_equal "2017-06-15", r.events[3].to_s
+    assert_nil r.events[4]
+  end
+
+  test "raises when :shift is true and :on is multiple days" do
+    assert_raises(ArgumentError) {
+      recurrence(
+        every: :month,
+        on: [1, 15],
+        starts: "2017-05-01",
+        until: "2017-06-30",
+        shift: true
+      )
+    }
+  end
 end