Why does Easter change every year?

The reason why Easter changes its date is because it was originally linked to the JEWISH Passover. The bible says that the death and resurrection of Christ happened around the time of this festival. The Jews used, and still use for religious purposes, a calendar where months are based on the cycle of the moon - each month started with a new moon - with occasional adjustments so that it doesn't drift too far away from the year indicated by the Sun. In particular, so that Passover (starting 14th Nissan) is always in the spring. Early Christians included many Jewish converts so, when they decided to celebrate the Resurrection of Christ, they chose to do so on the Sunday following Passover - the Sunday was chosen because it is the Lord's Day and every Sunday is considered a minor celebration of the Resurrection. By the 2nd century, the Romans had destroyed the Temple in Jerusalem and Jews no longer had a central authority to settle their calendar and decide how to adjust it. This meant that Jews in different parts of the world celebrated Passover on different dates. Christians wanted to celebrate Easter on the same date everywhere so they decided to adopt their own formula for working out when it should be, but based in the same idea of the Sunday after Passover. Jews did eventually agree their own common method of deciding the date of Passover but it did not match the Christian one so the link between Passover and Easter is not so close these days though in most years Easter still falls on the Sunday after Passover.

Every Year, Easter Sunday falls on any different dates between March 21st and April 25th.