How is the date of Easter calculated?

The first Sunday after spring begins AFTER the full moon.

When Easter is celebrated has nothing to do with anything christian. It moves around so much because it is based partly on a solar calendar and partly on the lunar calendar. The church decreed that Easter will be observed on the first sunday following the first full moon following the Vernal Equinox *unless* that sunday *is* the full moon in which case it will be observed on the following sunday. This was done to guarantee that Easter would never fall on the same day as a pagan celebration. Since pagans had a festival on the Vernal Equinox that was to be avoided. Many pagan group also would have a ritual celebration on ever full moon so that had to be avoided as well. The church did not want to risk Easter being celebrated on the same day as any pagan holiday because then they might not be able to tell which holiday it was that people were celebrating.

Here ya go. http://www.assa.org.au/edm.html

It's something to do with the closest Sunday to a full moon in March or April