In case you’re wondering how the date of Easter is calculated, you’ll find a wealth of information at Calendar & Easter Topics -- probably more than you want to know. Be warned that it’s written by an Australian who gets quite exercised about references to Easter being tied to the “vernal” equinox, because for him it's not. He's also unhappy about a certain lack of precision in the everyday way of talking about full moons.
Sadly, many definitions of Easter on the Internet and in Encyclopaedias and Almanacs are misleading, ambiguous and just plain wrong! This is obvious with the application of plain commonsense. A typical wrong definition is:How often does it happen that the filing deadline is the Monday after Easter? It happened 8 times in the 20th century (most recently 1990 and 1995) and will happen 8 times in the 21st century (2001), but on an irregular basis. It will happen again in 2017 and 2028, but after that your Easter weekend will be free of worries about Monday filing deadlines until 2063.
Easter Sunday is the Sunday following the full moon after the Vernal Equinox. This is wrong!
Vernal means spring, and countries in the Southern hemisphere have opposite seasons to those in the Northern hemisphere. Of course, Easter is not celebrated in September in the southern hemisphere! Most astronomers interpret "Vernal Equinox" to mean the March Equinox, but even that is equally wrong in this definition, but for different reasons (see below).
Also, I think that almost everyone reading this would assume that "full moon" refers to an astronomical full moon date. An astronomical full moon (AFM) occurs at one instant in time, and therefore occurs on 2 dates around the world (consider countries either side of the international dateline!). Again, countries do not celebrate different Easter dates based upon their own full moon dates!