Many people think that Christmas is the most important day in the Catholic liturgical calendar, but from the earliest days of the Church, Easter has been considered the central Christian feast. As Saint Paul wrote in 1 Corinthians 15:14, "If Christ has not been raised, then our preaching is in vain and your faith is in vain." Without Easter—without the Resurrection of Christ—there would be no Christian Faith. Christ's Resurrection is the proof of His Divinity.
Easter is not only the greatest Christian feast; it is the fulfillment of our faith as Christians. Through His Death, Christ destroyed our bondage to sin; through His Resurrection, He brought us the promise of new life, both in Heaven and on earth. Christ’s own prayer, "Thy Kingdom come, on earth as it is in Heaven," begins to be fulfilled on Easter Sunday.
That is why new converts are traditionally brought into the Church through the Sacraments of Initiation (Baptism, Confirmation, and Holy Communion) at the Easter Vigil service, on Holy Saturday evening.
Easter is a moveable feast, which means that it does not occur on the same date every year. The Council of Nicaea (A.D. 325) set the date of Easter as the Sunday following the paschal full moon, which is the full moon that falls on or after the vernal (spring) equinox.
We know that Easter must always occur on a Sunday, because Sunday was the day of Christ's Resurrection, why the paschal full moon? Because that was the date of Passover in the Jewish calendar, and the Last Supper (Holy Thursday) occurred on the Passover. Therefore, Easter was the Sunday after Passover.
The Church does not use the exact date of the paschal full moon but an approximation, because the paschal full moon can fall on different days in different time zones, which would mean that the date of Easter would be different depending on which time zone you live in. For calculation purposes, the full moon is always set at the 14th day of the lunar month (the lunar month begins with the new moon). Likewise, the Church sets the date of the vernal equinox at March 21, even though it can occur on March 20. Both approximations allow the Church to set a universal date for Easter.