Easter falls on the first Sunday after the first full moon on or after the spring equinox. The Council of Nicaea established this formula in 325 A.D. It continues to be used as the standard in Western Christianity into the 21st century.
The date for the full moon is set as the 14th day of the appropriate lunar cycle, and the date for the spring equinox is set as March 21.
The Eastern Orthodox religion celebrates Easter on a different date. The Orthodox church uses the older Julian calendar to set the date. The Julian calendar sets the spring equinox to April 3.