Use on in a period description

lib/crono/period.rb

@@ -20,6 +20,7 @@ module Crono
     def description
       desc = "every #{@period.inspect}"
       desc += format(' at %.2i:%.2i', @at_hour, @at_min) if @at_hour && @at_min
+      desc += " on #{DAYS[@on].capitalize}" if @on
       desc
     end
 

spec/period_spec.rb

@@ -9,8 +9,8 @@ describe Crono::Period do
 
   describe '#description' do
     it 'should return period description' do
-      @period = Crono::Period.new(2.day, at: '15:20')
-      expect(@period.description).to be_eql('every 2 days at 15:20')
+      @period = Crono::Period.new(1.week, on: :monday, at: '15:20')
+      expect(@period.description).to be_eql('every 7 days at 15:20 on Monday')
     end
   end