easter.rb

require 'time'

module EasterCalculations
  extend ActiveSupport::Concern
  
  module ClassMethods
    def easter(year = nil)
      y = year ||= now.year
      c = y / 100
      n = y - 19 * ( y / 19 )
      k = ( c - 17 ) / 25
      i = c - c / 4 - ( c - k ) / 3 + 19 * n + 15
      i = i - 30 * ( i / 30 )
      i = i - ( i / 28 ) * ( 1 - ( i / 28 ) * ( 29 / ( i + 1 ) ) * ( ( 21 - n ) / 11 ) )
      j = y + y / 4 + i + 2 - c + c / 4
      j = j - 7 * ( j / 7 )
      l = i - j
      m = 3 + ( l + 40 ) / 44
      d = l + 28 - 31 * ( m / 4 )
      mktime(y, m, d)
    end
  end
  
  def easter?
    self.class.easter.utc == self.at_beginning_of_day.utc
  end

  def good_friday?
    (self.class.easter.utc - 2.days) == self.at_beginning_of_day.utc
  end

  def easter_monday?
    (self.class.easter.utc + 1.day) == self.at_beginning_of_day.utc
  end
end

Time.send(:include, EasterCalculations)