It is the first Sunday following the first full moon, after the spring equinox.
In the time immediately following the death of Jesus, the lunar calendar of the Jewish religion and the solar, or Julian, calendar of the Romans caused confusion about the dates that events occurred, making it difficult to assign a specific date for religious celebrations. Adding to the difficulty, it seems that the date of Jesus’ resurrection was not recorded, apparently because Jesus’ followers expected Him to return soon, rendering an anniversary of the resurrection unnecessary.
At the Council of Nicea, some 300 years after the death of Jesus, it was decided that a date for Easter should be established. To accommodate both lunar and Julian calendars, an accord was reached: Easter would fall on the first Sunday, following the first full moon, following the vernal (spring) equinox.
As time passed, scientists realized that the Julian calendar did not correspond exactly with the actual length of the solar year, causing each Julian year to overflow slightly into the next. In the mid-1500s, Pope Gregory XIII had a Jesuit mathematician sort out the problem. He launched a new calendar, called the Gregorian, which introduced leap years to make up for the discrepancy. The Julian calendar, still used by some Eastern Orthodox churches, is now behind by about 13 days, which explains why Orthodox holidays fall on different dates.