Actually would you believe this is a hard question!!! In its simplist form it occurs around the time of Passover in the Jewish Faith. Passover falls on the 14th day of Nisan, which being that they have a lunar calendar is always a full moon.
Christians however had to problems with this. Number one it used a different length of year (342 vs 365) and it would shift days of the year. The official announcement as to what day Passover would be celebrated was also decided by the Jewsih Temple. This was yet another bitch in that another religion determined the date of your celebration.
so in 325 AD Constantine called the Council of Nicaea. Amongst aboiut a million other things (the true beginning of Catholic Dogma is the Council of Nicaea) it was decided that all Christians must celebrate Easter on the same day, the day must be a sunday, it must be with respect to the Jewish Passover as often as possible but would respect the Solar Calander rather than the lunar.
So in breif heres how its decided
Easter is the first sunday after after the first full moon after the vernal equinox (march 21). With the exception that if the full moon falls upon a sunday Easter is by default the sunday one week following as to not be on the same weekend as Passover.
Now this didn't work completely and there were some changes Pope Gregory 8th, for instance changed the callander but in the end the above rule is about as close as it gets.