isEmpty() determines if the Day is within the current month.
The Calendar_Month_Weekdays object is built with a wall calendar print format in mind, some of the days on the beginning of a month calendar are at the end of last month, and at the end of the month there are days shown from the next month.
Calendar_Month_Weekdays builds an array of days in a similar fashion.
isEmpty() determines if the day fetched is within the scope of the current month.