so easter is in march this year. the 'ecclesiastical' rules for calculating the date of easter sunday are: 1, easter falls on the first sunday following the first ecclesiastical full moon that occurs on or after the day of the vernal equinox. 2, this particular ecclesiastical full moon is the 14th day of a tabular lunation (new moon). 3, the vernal equinox is fixed as march 21. this page has more information and a code sample for calculating the date for any given year.