Why does the date of Easter vary from year to year?

The answer to this has to do with history and tradition. If you recall, Jesus and his disciples were Jewish and, as Jews, celebrated the Jewish holidays, among them the Feast of the Passover which commemorates the flight or Exodus of the Jews from Egypt. The Last Supper, which Jesus celebrated with his Disciples on what we now call Holy Thursday was the Passover meal that Jews celebrate at the start of Passover. According to the New Testament, Jesus was arrested that evening following the Passover meal, and was crucified on the next day which was Friday. He then rose the following Sunday. So we have the Last Supper on Thursday, the crucifixion on Friday and the resurrection on Sunday.

By the second century, the early Church had grown and expanded, with Christians in the West (Rome and Europe) celebrating Easter on Sunday, the day of the Resurrection, while those in the East (the Holy Land and Asia Minor) celebrating on the date of the start of Passover, which was the date on which Jesus had celebrated the Passover with his Disciples with the Last Supper and where he had instituted the Eucharist. There seems to have been a division along the lines of Jewish vs Gentile in this dispute.

The Christian Church began as a sect within the Jewish religion. However, the Apostles Peter and, especially, Paul directed their ministries toward the Gentiles while the Apostles John and Philip directed theirs more toward the Jews. The division over what day/date to observe Easter and whether the focus was to be on the institution of the Eucharist which coincided with the Jewish Passover meal or the Resurrection which occurred on Sunday was split along the lines of Jew and Gentile with each side claiming authority from either Peter and Paul (Gentile) or John and Philip (Jewish).

The Passover is celebrated on a specific date which is the fourteenth day of the Jewish month of Nisan. Since Christians used the Roman custom of measuring days from midnight to midnight, the date is the 14th, but for Jews, who measure a day from sunset to sunset, the Passover, which begins after sundown, the date is the 15th day of the month of Nisan. The controversy continued to smolder and to be debated at various church councils, but in time the champions of the Sunday celebration gained the upper hand and Sunday gradually became the day on which Easter was celebrated.

Even though Sunday became the accepted day for the celebration of Easter a problem still remained and that was the date itself. Again, the New Testament is very clear that the Crucifixion occurred during Passover and the date of Passover is the 14th/15th day of the Jewish month of Nisan. However, the Jewish calendar is a lunar/solar calendar and dates vary somewhat based upon the moon. So, the next task was to get the Churches throughout the world to celebrate Easter on the same Sunday each year.

At the Council of Nicaea, called by the Emperor Constantine in 325, it was formally decreed that all Churches would celebrate Easter on the same Sunday and that the Sunday would be the Sunday that followed the fourteenth day of the Pascal Moon. The Pascal Moon was decreed to be the new moon whose fourteenth day followed the Vernal or Spring Equinox. The necessary calculations for determining the Pascal Moon for each year was left up to the Church of Alexandria as Alexandria, Egypt was home to the best astronomers in the Roman Empire at that time.

Despite the Council's decree that all churches celebrate Easter on the same Sunday a new divergence occurred following the revision of the calendar in 1582 during the reign of Pope Gregory XIII. The new calendar, known as the Gregorian Calendar, was a revision to the Julian Calendar which had been created during the reign of the Roman Emperor Julius Caesar and was by then out of sync with the seasons. The Gregorian Calendar has been adopted by most nations of the world as their civil calendar. However, many Orthodox Churches continue to use the Julian Calendar as their ecclesiastical calendar which causes the dates of major Christian feast days, such as Christmas, Easter, Ash Wednesday, etc. to differ between the Eastern and Western branches of Christianity. While both branches of the Christian Church adhere to the same set of rules laid down at the Council of Nicaea, the application the rules to two different calendars continues to cause the dates to differ resulting in two different Sundays each year being celebrated as Easter Sunday.