How is this important date determined? In the first century Jesus and the early Christians accepted the determination of the date Nisan 14 (which commenced at sundown) as set by the Jewish temple priesthood in Jerusalem. It is noteworthy that Jesus celebrated the Passover meal on Nisan 14, as directed in the law of Moses. (Ex. 12:6-8; Lev. 23:5; Matt. 26:18-20) He did not eat the Passover meal on Nisan 15, as most Jews do today. After the destruction of the temple in 70 C.E., Christians had to determine the Nisan 14 Passover date themselves.

At the time when Roman Emperor Constantine made apostate Christianity the state religion (325 C.E.), the Council of Nicaea ordained that the celebration of Easter should always take place on the Sunday that immediately follows the full moon that happens upon, or next after, the day of the spring (vernal) equinox. Usually this equinox date is March 21. Should the fourteenth day from the new moon, which they regarded as the day of the full moon, fall on a Sunday, the celebration of Easter was deferred to the Sunday following. This was in order to avoid concurrence with the Jews and the minority of Christians, termed Quartodecimans, who still celebrated on the fourteenth of Nisan. In this way Christendom has come to have their “Maundy Thursday” always on a Thursday to commemorate Jesus’ Last Supper, and their “Good Friday” always on a Friday to commemorate his death.

At least by 1880 Jehovah’s anointed worshipers had departed from Christendom’s practice of celebrating the Lord’s Evening Meal several times a year and they observed it only on Nisan 14 after sundown. From then till about 1919 the anointed Christians accepted the dates as established by the Jewish calendar for the determining of Nisan 14. They realized that the Jewish calendar listed “Passover” for Nisan 15, after sundown. Nevertheless, these anointed Christians arranged to celebrate the Lord’s Evening Meal on the night of Nisan 14, even as did Jesus. Still, these Christians used the Jewish calendar in accepting the determination of the month of Nisan for each year.

The modern Jewish calendar determines the beginning of their month of Nisan by the astronomical new moon. However, usually it is eighteen hours or more later when the first sliver of the crescent of the new moon becomes visible in Jerusalem. Each year, in recent times, the governing body of Jehovah’s witnesses has determined the actual new moon that becomes visible in Jerusalem, which is the way the first of Nisan was determined in Biblical times. For this reason often there has been a difference of a day or two between the Memorial date of Jehovah’s witnesses and the Nisan 14 date according to the modern Jewish calendar.a

According to our present method of calculation, the Memorial date approximates the nearest full moon after the spring equinox. For example, in 1975 the Memorial date, as calculated fourteen days from the new moon (nearest the spring equinox) visible in Jerusalem, was Thursday, March 27, after sundown. Appropriately, there was also a full moon on Thursday, March 27, 1975. The date for Memorial in 1976, calculated by our present method, falls on Wednesday, April 14, after sundown. The full moon also occurs on this same date. So if, in the future, any of Jehovah’s people should be out of touch with the governing body, they could determine the Memorial date with fair accuracy from local calendars that show the first full moon after the spring equinox. The celebration would then take place after sundown of the day on which the full moon occurs.b