Holidays may have two origins, ecclesiastical and public/federal.
The ecclesiastical calendars of Christian churches are based on
cycles of moveable and immoveable feasts. Christmas, December 25th,
is the principal immoveable feast. Easter is the principal moveable
feast, and dates of most other moveable feasts are determined with
respect to Easter.
The date of Easter is evaluated by a complex procedure whose detailed
explanation goes beyond this description. The reason that the calculation
is so complicate is, because the date of Easter is linked to (an
inaccurate version of) the Hebrew calendar. But nevertheless a short
answer to the question "When is Easter?" is the following: Easter
Sunday is the first Sunday after the first full moon after vernal
equinox. For the long answer we refer to Toendering (1998).
The algorithm computes the date of Easter based on the algorithm of
Oudin (1940). It is valid for any Gregorian Calendar year.