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I've just noticed that the Jewish Festival of Passover falls on Sunday 28th March (2021), the night of the full moon, but the Christian Church is celebrating "Good Friday" on Friday 2 April (2021).
So the Christian celebration of Easter 2021 is a week late? Why?
Why is the Jewish Passover on 28 March 2021 but Good Friday is on 2 April 2021?
Actually everything is in its’ proper place.
Easter is a “movable feast” and does not have a fixed date. However, it is always held on a Sunday between March 22 and April 25.
Over a 500-year period (from 1600 to 2099 AD), it just so happens that Easter will have most often been celebrated on either March 31 or April 16.
Easter is celebrated on the first Sunday following the full Moon that occurs on or just after the spring equinox.
According to a Fourth Century ruling, of the Council of Nicea, the date of Easter is set for the first Sunday following the Paschal Full Moon, which is the first full Moon of Spring, occurring on or shortly after the vernal equinox, or spring equinox. March 22 is the earliest Easter can occur on any given year, and April 25 is the latest. If that first spring full Moon occurs on a Sunday, then Easter will be observed the following Sunday.
The Date Of Easter And Its Controversies
Fixing the date on which the Resurrection of Jesus was to be observed and celebrated triggered a major controversy in early Christianity in which an Eastern and a Western position can be distinguished. The dispute, known as the Paschal controversies, was not definitively resolved until the 8th century. In Asia Minor, Christians observed the day of the Crucifixion on the same day that Jews celebrated the Passover offering—that is, on the 14th day of the first full moon of spring, 14 Nisan (see Jewish calendar). The Resurrection, then, was observed two days later, on 16 Nisan, regardless of the day of the week. In the West the Resurrection of Jesus was celebrated on the first day of the week, Sunday, when Jesus had risen from the dead. Consequently, Easter was always celebrated on the first Sunday after the 14th day of the month of Nisan. Increasingly, the churches opted for the Sunday celebration, and the Quartodecimans (“14th day” proponents) remained a minority. The Council of Nicaea in 325 decreed that Easter should be observed on the first Sunday following the first full moon after the spring equinox (March 21). Easter, therefore, can fall on any Sunday between March 22 and April 25.
The first full moon after the spring equinox of 2021 falls on March 28th, so everything is absolutely normal.
Easter never falls on the exact date of the first full moon of Spring, so as to avoid being celebrated at the same time as the Jewish Passover.
Long ago at Nicea it was decided not to permit things Christian (Good Friday, resurrection Sunday) to coincide with things Jewish (Passover, full moon), as the two could fall on the same day. So, the rule of calculating the date of Easter became to observe it on a Sunday after the the first full moon after the Spring Equinox. The idea was to observe the day, rather than the date.
It was declared to be particularly unworthy for this, the holiest of all festivals, to follow the custom [the calculation] of the Jews, who had soiled their hands with the most fearful of crimes, and whose minds were blinded. In rejecting their custom,113 we may transmit to our descendants the legitimate mode of celebrating Easter, which we have observed from the time of the Saviour’s Passion to the present day [according to the day of the week]. From Emperor's Letter
So, to answer the OP, Easter is observed on the Sunday a week after the full moon because of this tradition.
The Rabbinic Jewish month of Nisan 5781 begins this year on March 14 (that is, at sunset on March 13). The Feast of Unleavened Bread is on its 15th day, hence March 28th.
The Gregorian month of Nisan begins this year on March 15 (that is, at sunset on March 14). Easter is always the third Sunday in Gregorian Nisan, hence April 4th. Since Easter is always the third Sunday, the age of the moon on Easter must be greater than or equal to 15 days and less than or equal to 21 days. This year the age of the moon on Easter will be 21 days, the latest it can be in its lunar month. So we will not have a full-looking moon for our night services on Saturday night.
