In contrast to Christmas, which always takes place on a fixed day, Easter has always been a movable feast. Christian Easter and the Jewish Passover are intimately linked and their calculation is very similar - the differences are now largely due to the differences in the Gregorian and Hebrew calendars but the fundamental ideas remain the same. Indeed, in most languages other than English the word for Easter is a derivation of Passover - the latinate adjective 'paschal' illustrates this link.
To calculate the date on which Easter falls using the Western Gregorian calendar is relatively straight-forward. Firstly, take the 21st March as the ecclesiastical vernal equinox. Then find when the first full moon occurs after this date; yet again, for calculation purposes this is taken as the 14th day of the lunar month. Then just find the first Sunday after this full moon: that will be Easter Sunday.
If one dispenses with the above protocols and uses up to date astronomical data then Easter will still fall on the same day for the vast majority of years as the Gregorian calendar adjusts itself to keep as closely as possible to the solar cycle.
I will pass over the theological links between Judaism and Christianity, save to say that some form of spring celebration has always been held, even in pagan times. The Anglo-Saxon word for Easter, being derived from a fertility goddess, and also the root of the name of hormone oestrogen, illustrates how Christian Europe adapted a celebration that already existed. After all, bunny rabbits and eggs seem to have little to do with Christianity and a lot to do with fertility.
In 2010, Easter Sunday will fall on April 4 for both Western and Eastern churches. This year, 2009, Easter is on April 12 for Western Christianity and on April 19 for the Orthodox Churches.