Since Easter in various churches was celebrated on various dates, Emperor Constantine convened in 325 on the first assembly in Nicaea to discuss this issue and some solutions were made. But the rules governing the date of Easter today, weren't defined until 526 year.
According to these rules Easter is always after the spring equinox in the first week after the full moon. It is always on Sunday, but must not coincide with the day when the Jews celebrate the Passover, nor to be before that day.

If the date falls before Passover, Easter is postponed for the first week after the next full moon, and if they coincide on the same day, it is postponed for the next week. Easter can fall in 35 dates from 4th of April to 8th of May, which means two days after St. George's Day (Gjurgjovden).

The next Easter is on April 19th.