Easter commemorates the resurrection of Jesus Christ, although it is not celebrated on the actual anniversary of the resurrection.  Among Christians in the West it may fall on any date between March 22 and April 25.  Eastern Christianity observes Easter between April 4 and May 8.  There have been efforts to have all Christians observe Easter on the same day, and other efforts to hold it on the same day each year.  However, these efforts have not been successful.

FYI:  Summer Time Begins in Europe on March 27th.