This Is Why Easter's Date Is Always Different

Easter, the day Christians commemorate the resurrection of Jesus Christ, is observed on the first Sunday after the “Pascal Full Moon” (the first full moon of spring) following the spring equinox. That day always occurs on March 21, according to a decree by the early Christian Church at the Council of Nicaea in 325 A.D. and the Gregorian calendar introduced by Pope Gregory in 1582.

Therefore Easter can fall on any Sunday between March 22 and April 25. That is also why Easter and church holidays leading up to that day, like Ash Wednesday, Palm Sunday and Good Friday, are referred to as “moveable feasts.”

But the Eastern Orthodox churches did not adopt the Gregorian calendar method of determining Easter’s date and instead follows the Julian calendar, a solar calendar adopted by Julius Caesar in 45 BC. The Julian calendar is 13 days ahead of the Gregorian calendar, meaning “the Orthodox Easter celebration usually occurs later than that celebrated by Protestants and Roman Catholics,” according to Encyclopedia Britannica. “The Orthodox tradition prohibits Easter from being celebrated before or at the same time as Passover.”

See the Most Surreal Easter Fireworks Display in the World

A truck waits to transport 8,000 homemade rockets down to the village after being stored in hiding for the past few months in Vrontados, Chios, Greece.

Marco Kesseler

The church of Panageia Erithiani in Vrontados, Chios, Greece.

Marco Kesseler

A boy shows some of his rockets stored in a hidden building.

Marco Kesseler

The frontline launch area of Agios Markos in Vrontados, Chios, Greece. Each parish has several factions and firing ranges, coming together to fire tens of thousands of rockets.

Marco Kesseler

A young boy carries a sack of rockets down to the firing range.

Marco Kesseler

Team Aeolos load 27,500 rockets on a lorry from their hidden storehouse in the mountains.

Marco Kesseler

At dusk the first rockets are fired at the churches.

Marco Kesseler

Agios Markos fire their rockets across the valley, towards the church of Panageia Erithiani in Vrontados, Chios, Greece.

Marco Kesseler

Rockets are lit from one of the factions supporting Agios Markos.

Marco Kesseler

Rockets rain down on the church of Panageia Erithiani.

Marco Kesseler

A rocket is lit at a small fishing port in Vrontados.

Marco Kesseler

Team Aeolos light their rockets.

Marco Kesseler

Rockets rain down on the village of in Vrontados, Chios, Greece.

Marco Kesseler

Barricades to the church of Panageia Erithiani.

Marco Kesseler

Some of the spent rockets that hit the church lie on the ground.

Marco Kesseler

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Write to Olivia B. Waxman at olivia.waxman@time.com.