holiday.rb

# in Norway
module Calendar
  module Holiday

    FEBRUARY = 2
    NOVEMBER = 11

    def mothersday(year)
      second_sunday_in(FEBRUARY, year)
    end

    def fathersday(year)
      second_sunday_in(NOVEMBER, year)
    end

    def next_mothersday(date = Date.today)
      next_holiday_after(date) { |year| mothersday(year) }
    end

    def next_fathersday(date = Date.today)
      next_holiday_after(date) { |year| fathersday(year) }
    end

    private

    def next_holiday_after(date)
      this_years = yield(date.year)
      if this_years >= date
        this_years
      else
        yield(date.year + 1)
      end
    end

    def second_sunday_in(month, year)
      # Day number 8 always falls within
      # the second week of the month
      second_week = Date.new(year, month, 8)
      second_week + days_until_sunday(second_week)
    end

    def days_until_sunday(date)
      (7 - date.wday) % 7
    end

  end
end

holiday_spec.rb

require 'rspec-given'
require './holiday'

RSpec::Given.use_natural_assertions

describe Calendar::Holiday do
  include Calendar::Holiday

  Given(:this_date) { Date.parse("2013-05-03") }

  Then { mothersday(2013) == Date.parse("2013-02-10") }
  Then { mothersday(2014) == Date.parse("2014-02-09") }

  Then { next_mothersday(this_date) == Date.parse("2014-02-09") }

  Then { fathersday(2012) == Date.parse("2012-11-11") }
  Then { fathersday(2013) == Date.parse("2013-11-10") }

  Then { next_fathersday(this_date) == Date.parse("2013-11-10") }
end