Along with a little information about the Gregorian Calendar, Tony Worner provides a brief explanation about how the dates of Easter are set each year:

The current secular calendar or Gregorian calendar is a ‘solar calendar’ used in most of the world today. It was introduced in October 1582 by Pope Gregory XIII and was a reform of the Julian calendar which was proposed by Julius Caesar in 46 BC.

It was at this time, with Pope Gregory, that the date of Easter was finally determined to be followed by the Catholic (Christian) Church. Eastern Orthodox Catholics still follow the Julian calendar.

Specifically, Easter is celebrated on the first Sunday following the full Moon that occurs on or just after the Spring Equinox (in the Northern Hemisphere). There is a caveat to this but to save confusion we’ll leave it at that!

Hence in any given year, the earliest Easter can occur is March 22 and April 25 is the latest.