Passover is a celebration that varies from year to year in relation to our calendars, because it is based on the Jewish calendar that is guided by the moon. The central day to calculate the Passover and Lent is the day of the Resurrection of the Lord. In order to calculate Lent, which begins on Ash Wednesday, we must count 40 days back from the Sunday previous to the Passover Resurrection Sunday, that is, Palm Sunday. After the Passover Resurrection Sunday 40 days are counted until the Ascension (although the feast is celebrated on the following Sunday); Pentecost is celebrated a week after the Ascension (although it occurs 50 days after the Resurrection, according to the Bible). How is the Passover Resurrection date calculated?

The Catholic Church wanted from the beginning, to commemorate the death of Jesus on the same day according to the gospel narratives. It has been a Jewish tradition to celebrate their Passover using the Lunar Calendar; of course at the time of Jesus, Passover was determined by the moon cycles. Starting off of this, the Easter Calendar goes from the day of the Resurrection, which is the next Sunday right after the Full Moon of the month of Nissan (the month of the Jews) that goes from March 22 to April 25. In other words, this is the Sunday that follows the first Spring Full Moon (Spring usually begins on March 21).

The Council of Nicea (325), approved this way of calculating the day of the Resurrection of the Lord, making sure that the Jewish Passover did not fall on the same date of the Christian Passover.