monthly.rb

class Monthly
  def initialize(date = Date.today)
    @date = date
  end

  def first
    Date.civil(@date.year, @date.month, 1)
  end

  def last
    date = first
    date += 1 until date.succ.day == 1
    date
  end

  def first_on_calendar
    date = first
    date -= 1 until date.wday == 0
    date
  end

  def last_on_calendar
    date = last
    date += 1 until date.wday == 6
    date
  end

  def calendar_dates
    dates = []
    first_on_calendar.upto(last_on_calendar) {|d| dates << d}
    dates
  end
end

monthly_spec.rb

require 'timecop'
require 'monthly'

describe Monthly do
  describe 'April 2009' do
    before do
      Timecop.freeze(Date.civil(2009, 4, 16)) do
        @monthly = Monthly.new
      end
    end

    it 'should give april 1 for the first day' do
      @monthly.first.should == Date.civil(2009, 4, 1)
    end

    it 'should give april 30 for the last day' do
      @monthly.last.should == Date.civil(2009, 4, 30)
    end

    it 'should give march 29 for the first day on calendar' do
      @monthly.first_on_calendar.should == Date.civil(2009, 3, 29)
    end

    it 'should give may 2 for the last day on calendar' do
      @monthly.last_on_calendar.should == Date.civil(2009, 5, 2)
    end

    it 'should give the correct set of dates' do
      dates = %w(
        29 30 31  1  2  3  4
         5  6  7  8  9 10 11
        12 13 14 15 16 17 18
        19 20 21 22 23 24 25
        26 27 28 29 30  1  2
      ).map {|s| s.to_i}
      @monthly.calendar_dates.map {|d| d.day}.should == dates
    end
  end

  describe 'October 2008' do
    before do
      @monthly = Monthly.new(Date.civil(2008, 10, 20))
    end

    it 'should give october 1 for the first day' do
      @monthly.first.should == Date.civil(2008, 10, 1)
    end

    it 'should give october 31 for the last day' do
      @monthly.last.should == Date.civil(2008, 10, 31)
    end

    it 'should give september 28 for the first day on calendar' do
      @monthly.first_on_calendar.should == Date.civil(2008, 9, 28)
    end

    it 'should give november 1 for the last day on calendar' do
      @monthly.last_on_calendar.should == Date.civil(2008, 11, 1)
    end

    it 'should give the correct set of dates' do
      dates = %w(
        28 29 30  1  2  3  4
         5  6  7  8  9 10 11
        12 13 14 15 16 17 18
        19 20 21 22 23 24 25
        26 27 28 29 30 31  1
      ).map {|s| s.to_i}
      @monthly.calendar_dates.map {|d| d.day}.should == dates
    end
  end
end