Christmas is always on the 25th of December.  Thanksgiving is the third Thursday of the month of November.  The dates of most holidays hold to some established criteria of date placement on the calendar.  At first glance, it appears that Easter doesn't follow this rule.  Some years it occurs in March - in others it happens in April.  And the dates of the Easter holiday seem to fall all over the place.  However, at closer look - Easter (and the period of Lent that precedes it) is also as predictable as some of the other holidays.

So how is date of Easter Sunday determined?

The answer to this puzzle involved the moon.  According to Catholic.com, Easter Sunday is the first Sunday that follows the Paschal full moon - that is the first full moon after the spring equinox.  If that first full moon occurs on a Sunday, the following Sunday is celebrated as Easter Sunday.  Looking at the phases of the moon and comparing them to the calendar, Easter will always fall somewhere between March 22 and April 25.

Now that you know what date Easter will occur, you can determine the rest of the dates of Lent by simply backing up your calculations.  Palm Sunday?  That's the Sunday before the date of Easter. Ash Wednesday?  Take the date of Easter, skip back six weeks and go back four more days to that Wednesday;  that's the date of Ash Wednesday - the day that Lent starts.