Easter and the holidays that are related to it are so called moveable feasts, because they do not fall on a fixed date on the calendar. Instead, the date for Easter is determined on a lunisolar calendar similar to the Hebrew calendar. The First Council of Nicaea of the Catholic Church (325 AD) established the date of Easter as the first Sunday after the full moon following the March equinox. Ecclesiastically, the equinox is March 21st when the "full moon" is not necessarily the astronomically correct date.
In Western Christianity, Easter always falls on a Sunday between March 22 and April 25th or within appx. seven days after the astronomical full moon. The following day, often referred to as Easter Monday, is a legal holiday in many countries with predominantly Christian traditions.
The New Testament teaches that the resurrection of Jesus, which Easter celebrates, is a foundation of the Christian faith. Easter is linked to the Passover through the Last Supper and crucifixion that preceded the resurrection. According to the New Testament, Jesus prepared himself and his disciples for his death in the upper room during the Last Supper. He identified the matzah and cup of wine as his body soon to be sacrificed and his blood soon to be shed for Christ (the Passover lamb).
One interpretation of the Gospel of John is that Jesus, as the Passover lamb, was crucified at roughly the same time as the Passover lambs were slain in the temple. Easter is the principal feast of the ecclesiastical year. It is sometimes referred to as the greatest feast for which Christmas is celebrated only in preparation for Easter. The order of Sundays from Septuagesima; to the last Sunday after Pentecost; from the Prayer of Jesus in the Garden; to the feast of the Sacred Heart; all depend upon the Easter date. It is also the oldest feast of the Christian Church, as old as Christianity, and can also be considered one of the best links between the Old and New Testaments.
