Easter is the celebration of Christ's resurrection from the dead. It is celebrated on Sunday, and marks the end of Holy Week, the end of Lent, the last day of the Easter Triduum (Holy Thursday, Good Friday and Easter Sunday), and is the beginning of the Easter season of the liturgical year.
As we know from the Gospels, Jesus Christ rose from the dead on the third day following his crucifixion, which would be Sunday. His resurrection marks the triumph of good over evil, sin and death. It is the singular event which proves that those who trust in God and accept Christ will be raised from the dead.
Since Easter represents the fullfilment of God's promises to mankind, it is the most important holiday on the Christian calendar. In the Gospels, the precise details of the Easter narrative vary slightly, but none of these variances are critical to the main story. In fact, it is argued that the variances are simply matters of style and not substance. Despite the variances, the key aspects of the Easter story all match. Above all, they agree that the tomb of Christ was indeed empty, which is the most essential fact. Based on direct evidence from the mid-second century, it is believed that Easter was regularly celebrated from the earliest days of the Church. Following Easter Sunday, the season of Easter begins and lasts for seven weeks, ending with Pentecost.