WASHINGTON — Easter's Date Changes Yearly - Here's The Basics
Easter is determined as the first Sunday after the first full moon after the vernal equinox. Easter can be no earlier than March 22 and no later than April 25. 

Wednesday, the vernal equinox occurred at 5:58 PM and the full (worm moon) moon at 9:43 PM.

So Why Isn't Easter This Sunday?

The Answer Dates Back Thousands Of Years
The answer goes back to the ancient Greeks and a mathematician named Meton. He came up with the Metonic lunar calendar which illustrates moons following a 19 year cycle. In his calendar and Gregorian Calendar the Vernal Equinox is always March 21st. So, the next full moon (pink moon) is April 19th so Easter is then Sunday, April 21st. We still use the Metonic calendar to determine eclipses. Not bad for a calendar created over 2300 years ago !

For More, check out the US Naval Observatory's info on Easter

Provides details on the calculation of the date of Easter in the Gregorian Calendar.