The Catholic Church determines the date based on the fact that Christ’s death occurred near the Jewish Passover, which is stated in the Gospel accounts of the Last Supper, when Jesus gathered with the Apostles to celebrate the feast commemorating the Exodus of the Jewish people from slavery in Egypt.
According to the Mosaic Law, the Jewish people must renew this celebration each year on the 15th of the month of Nisan, which begins with the first new moon of Spring.
In the early Christian world, there were varying dates to celebrate Easter. Eventually, the Church moved to settle on a single date for Easter.
The First Council of Nicea, held in 325, decided that Easter would be celebrated on the first Sunday after the full moon following the spring equinox, making the earliest possible date for Easter March 22 and the latest possible April 25.
Most of the time Easter falls during the first week or two of April.
Today, Orthodox Christians use the Julian calendar to calculate the Easter date instead of the Gregorian calendar, which was introduced in 1582 and is used by most of the world. Because the Julian calendar calculates a slightly longer year, it is currently 13 days behind the Gregorian calendar.