Easter Day is the final 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. Easter represents the fulfillment of God's promises to mankind and is the most important holiday on the Christian calendar.
After the sabbath, as the first day of the week was dawning, Mary Magdalene and the other Mary went to see the tomb. And suddenly there was a great earthquake; for an angel of the Lord, descending from heaven, came and rolled back the stone and sat on it. His appearance was like lightning, and his clothing white as snow. For fear of him the guards shook and became like dead men. But the angel said to the women, "Do not be afraid; I know that you are looking for Jesus who was crucified. He is not here; for he has been raised, as he said. Come, see the place where he lay.