It's a religious holiday from the Christian tradition - those people who believe that Jesus was the son of God and became human, and then died and rose from the dead before ascending to heaven. It shares a common origin with Judaism & Islam - two other traditions from Earth that was.
Easter is perhaps the greatest holiday of the tradition, when believers celebrate his death and resurrection. The service will have some readings that tell the story of what supposedly happened, and some songs as well, plus a short sermon by me.