Christians around the world Sunday celebrate Easter — the day they believe Jesus rose from the dead. It is the holiest day of the Christian calendar.

Easter is Christianity’s “moveable feast,” falling on a different date each year. Western Christian churches celebrate Easter on the first Sunday following the full moon after the vernal equinox.

Easter marks the end of Holy Week, which is the week before Easter and includes Maundy Thursday, the day of Jesus’ last supper with his disciples, and Good Friday, the day Jesus was crucified.

Authorities in Egypt have beefed up security this year for Easter after a suicide bomb attacks on a Coptic Christian church last Sunday left dozens dead and more than 100 wounded.