While many people most readily associate the Christmas season with Christianity, some followers actually hail Easter Sunday as the most important celebration in the religion — and it's coming up.

On Easter Sunday, Christians celebrate the resurrection of the faith's founder, Jesus Christ, and its date changes every year for a similar reason that the Islamic holy month of Ramadan shifts annually: Jews and Muslims historically used calendars based on phases of the moon. Early Christians celebrated Easter on different days around the world until the Council of Nicaea in 325 established the practice of observing Easter on the Sunday after the first full moon of the spring equinox, during which both day and night are of equal length.

In the U.S., this year's Easter Sunday falls on April 16 for Roman Catholics. Last year, it was celebrated two weeks later, on March 27.

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That's because the movements of the moon don't exactly line up with the Gregorian calendar that was later adapted in the 16th century and based on the cycles of the sun. Because Jesus was killed and resurrected around the time of the Jewish feast of Passover, according to Christian beliefs, the church wanted the two religious holidays to roughly coincide, which meant trying to reconcile both the lunar and solar calendars. Eastern Orthodox Christians follow the Julian Calendar, the predecessor to the Gregorian calendar that varies by about 13 days. This year, however, Easter Sunday coincidentally falls on the same date for both Western and Eastern sects of the church.

Easter Sunday comes as the Christian holy season of Lent ends. Lent began March 1 this year with the celebration of Ash Wednesday. Christians believe that Jesus spent 40 days fasting in the desert and many believers devote the season to prayer and sacrifice before observing Jesus' Last Supper on Holy Thursday, his death on Good Friday and his resurrection on Easter Sunday.

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