Easter is the highest, greatest, holiest celebration of the entire year. Easter is the remembrance of the Resurrection of Jesus Christ from the dead and is the sole reason that the whole of Christianity exists. St. Paul says as much when he writes:
If Christ has not been raised, your faith is futile and you are still in your sins.
1 Corinthians 15:17
For St. Paul, and indeed for every Christian, the Resurrection of Jesus Christ is the hinge upon which everything else either stands or falls.
Easter is more than just one day; it the season that immediately follows Lent and Holy Week. As Lent lasts for 40 days, Easter is celebrated for 50 days, recalling the 50 days that passed from the resurrection of Jesus from the dead until the coming of the Holy Spirit on Pentecost.
The Easter Vigil Mass, celebrated in the evening on the Saturday before Easter Sunday, is the most important liturgical celebration in the Church. At this Mass, all of the symbols of the Church are on full display and those joining the Church recieve the Sacraments for the first time as the Church remembers that first Easter morning, when the women who followed Jesus arrived at His tomb but found it empty.