There are fourteen different calendars. To see why, consider the day
of the week on which January 1 falls. It can fall on a Sunday, Monday,
Tuesday, or any of the seven days of the week. You'll need seven
different calendars to handle each of these situations.

But there are also leap years, so you'll need seven different
calendars to handle the seven different days of the week on which
January 1 can fall, when the year is a leap year. These calendars are
different from the ones above, since these have 366 days while the
non-leap year calendars have the usual 365 days.

Since every year is either a leap year or it is not, these fourteen
cases are all you need.