Christians visiting the Holy Land in the spring sometimes fail to appreciate the link between Passover and Easter: Jesus came to Jerusalem in April circa 34, making his triumphal entry on the Sunday of the last fateful week of his life, in order to offer a Passover sacrifice at Herod’s magnificent newly-built Temple.
He celebrated the Passover seder feast that Thursday night, an event commonly referred to as the Last Supper. Returning with his apostles to their encampment at Gethsemane on the nearby Mount of Olives, he was arrested that evening after being betrayed by Judas.
On Friday, the holy day of Passover, Jesus was tried and then crucified. His corpse was hurriedly placed in a new sepulcher or family tomb belonging to Joseph of Arimathea near to the Skull Hill execution grounds (believed by some to be located adjacent to what is known as the Garden Bomb) to so as not to violate the Sabbath that began Friday shortly before sundown. Sunday morning it was discovered that the rolling stone sealing Jesus’ tomb had been shifted, and the sepulcher was empty. Jesus had arisen.
When it came to actually specifying the date in which Easter would be celebrated annually, the Church fathers wanted the holiday to closely follow Passover, after all, that was when Christ died. But interestingly, in determining the date of Easter, Christianity did not make Easter’s date dependent on Passover and, in fact there are years when Easter falls almost a full month in advance of Passover. Why?
This is because Judaism follows a lunar calendar comprising twelve lunar months of 29 to 30 days in length with the new moon marking the beginning of each month and the full moon occurring halfway through the month. Because the lunar calendar is shorter than the solar calendar, over time the Jewish calendar falls out of line with the seasons which is why an additional month is added to the Jewish calendar very few years.
Western Catholics, including Roman Catholics and Protestants, celebrate Easter on the Sunday immediately following the paschal full moon that occurs on or after the spring equinox. These dates are fixed in advance according to the Ecclestiastical full moon schedule that was set in 1583A.D. and can vary from the date of the Paschal full moon by up to two days. If the full moon falls on a Sunday, then Easter is the following Sunday. It can fall anywhere March 22 and April 25.
The Eastern or Orthodox Churches base their holiday calculations on the Julian calendar rather than the revised Gregorian calendar, adopted by Pope Gregory XIII in 1582. Unlike the Gregorian calendar, The Julian calendar does not take into account the extra day every fourth leap year. Consequently, both the Western and Eastern churches only occasionally celebrate Easter on the same day.