You would know this if you had paid attention in your astronomy class.
Easter is a so-called moving feast of the Christian church: in the year 325 the first synod of Nicea decided that in every year the first Sunday after the first full moon following the spring equinox we celebrate the Easter Sunday. Why the bunny? In the old, pagan times, this time of the year was the rebirth the nature and the feast of fertility, somewhere around the spring equinox.
The dates of Easter are tied to the full Moon and the vernal equinox (not the civil calendar).
- Christian churches that follow the Gregorian calendar, such as many Western Christian churches, celebrate Easter on the first Sunday after the paschal full Moon on or just after the vernal equinox. For simplicity, the equinox is considered March 21. The paschal full Moon always falls on the 14th day of a lunar month; because ancient calculations did not take into account certain lunar motions, this date may be slightly off from the astronomical full Moon date.
- In Christian churches that follow the Gregorian calendar for determining the date of Easter, the observance can never occur before March 22 or after April 25. In Christian churches that follow the Julian calendar for determining the date of Easter, the observance can occur between April 4 and May 8 (using Gregorian calendar dates).