This year, if you have not already noticed, Easter is going to arrive rather early: Sunday, March 27. The date of Easter has a curious link to the moon’s phases.
In fact, Easter will slip into April for the next eight years. Not until the year 2024 will Easter once again occur in March.
The new moon nearest the vernal equinox is the first day of the Hebrew month Nisan. Then comes the Paschal (full) moon, also known as the Paschal Term, which may fall on the 14th or 15th of Nisan, which also marks Pesach, or Passover.
Traditionally, Easter is observed on the Sunday after the Paschal Term. If the Paschal Term occurs on a Sunday, Easter is moved to the following Sunday. Following these rules, we find that Easter can fall as early as March 22 and as late as April 25. Pope Gregory XIII decreed this in 1582 as part of the Gregorian calendar.