In a rare occurrence, both major branches of Christianity are celebrating Easter, the holiest day in the Christian calendar, on the same day today. Eastern Orthodox and western Christians usually observe the holy day on different Sundays, a function of calendar dynamics and different traditions. This year, the Gregorian and Julian calendars have converged, creating a single celebration of the day Christians believe Jesus Christ rose from the dead.

Western Christians, including Catholics and Protestants, celebrate Easter according to the later Gregorian calendar - instituted by Pope Gregory 13 in 1582. Orthodox Christians celebrate Easter according to the old Julian calendar, named after Julius Caesar - who established it in 46 B.C.

From Athens to Singapore and Australia to Siberia, Christians are celebrating the resurrection of the son of God. In Jerusalem, a sea of candles and torches at midnight illuminated the Church of the Holy Sepulcher, which tradition holds was the site of Christ's tomb.

At the Vatican, tens of thousands are expected in Saint Peter's Square for the pope's Easter Sunday Mass, when he delivers a message of peace and blessing to the world. He baptized several converts to Catholicism Saturday night at a vigil mass in St. Peter's Basilica.

This year's celebrations come with exceptionally tight security. With memories still fresh of the March 11th train bombings in Madrid and the September 11, 2001 terrorist attacks in the United States, the date, April 11, and various terrorist threats have caused governments to tighten security at churches, mosques and synagogues.