Easter is the most important festival in the Christian calendar. It celebrates the resurrection from the dead of Jesus, three days after He was executed.
After Jesus was crucified on the Friday (now known as Good Friday), His body was taken down from the cross, and buried in a cave tomb and a stone rolled to seal the tomb.
On the Sunday, Mary Magdalene and some other women came and found that the stone had been moved and the tomb was empty.
Jesus himself was seen that day by Mary and the disciples, and for forty days afterwards by many people. His followers realised that God had risen Jesus from the dead. Christians call this the Resurrection and this is why Easter is a really big celebration to Christians because He rose from the dead after dying for our sins.