Easter Sunday is celebrated each year to remember the resurrection of Jesus Christ. It is the biggest festival for Christians all around the world. There is no fixed date for Easter and the Easter Sunday calendar is formulated on the basis of a few factors like march equinox and Paschal Full Moon.
The date for Easter Sunday and the festivals which are related to Easter and determined its date are not set in the Gregorian or the Julian calendar. Instead a well-defined rule is followed for formulating the Easter Sunday calendar.
According to Christian Prayer Books, Easter Sunday is always the first Sunday succeeding the Paschal Full Moon.
The Paschal Full Moon date is a worldwide event which differs from the Astronomical Full Moon Date by two or three days.
March Equinox (which was set on March 20) must always precede or coincide with the date of Paschal Full Moon and hence, Easter Sunday.