Added following failing spec FIRST

    context "#weekday_name_yesterday" do
        it "returns yesterday's week-day name" do
            calendar = Calendar.new
            expect(calendar.weekday_name_yesterday).to eql( (Time.now - Calendar::SECONDS_IN_ONE_DAY).strftime('%A') )
        end
    end

    context "#weekday_number_yesterday" do
        it "returns yesterday's week-day number" do
            calendar = Calendar.new
            expect(calendar.weekday_number_yesterday).to eql( (Time.now - Calendar::SECONDS_IN_ONE_DAY).wday )
        end
    end

Then added the simplest implementation in Calendar class to pass the above mentioned failing specs.