Gemfile

# frozen_string_literal: true

source 'https://rubygems.org'

gem 'activesupport'
gem 'minitest'
gem 'minitest-reporters', '>= 0.5.0'
gem 'rr'
gem 'rubocop'

holiday.rb

# frozen_string_literal: true

class Holiday
  def sayHello
    if christmas?(Date.today)
      "Merry X'mas"
    else
      "Today is not X'mas"
    end
  end

  private

  def christmas?(date)
    date.day == 25 && date.month == 12
  end
end

holiday_test.rb

# frozen_string_literal: true

require 'minitest/autorun'
require 'date'

require_relative 'holiday'

class HolidayTest < Minitest::Test
  def test_has_class_and_method
    assert Holiday.new.sayHello
  end

  def test_is_christmas
    Date.stub(Date.today, Date.new(2020, 12, 25)) do
      assert_equal "Merry X'mas", Holiday.new.sayHello
    end
  end

  def test_is_not_christmas
    Date.stub(Date.today, Date.new(2020, 3, 3)) do
      assert_equal "Today is not X'mas", Holiday.new.sayHello
    end
  end
end