Refactor zellerCongruence(), calculateSubstituteHolidays() and calculateNationalHolidays().
Change the view part way through.
ToDo:
* Detach holidays member from SimpleCalendar.
* Change the controller to help the view.
Change class SimpleCarendar to the view-friendly one.
Change the view.
Add two tests of isSunday and isSaturday.
Move toString implementations of class Month and Day to object Month and Day, respectively.
ToDo:
* Add links of Next Month and Previous Month in the view templete.