Why Holy Week has no fixed date unlike Christmas and other holidays?

Holy Week traditions in Philippines, photo from Rappler

Unlike most church holidays, the Holy Week is a celebration that has no permanent date. It changes every week but it is based on Easter, a Catholic celebration that is for and marks the resurrection of Jesus Christ.

Easter’s date isn’t definite though, the feast moves around the Jewish Passover. Jewish celebrations by the way are also movable due to the Jew’s utilization of lunisolar liturgical calendars, a calendar that follows both lunar and solar cycles.

History

2-4 AD

• The diocese from the 2nd to the 4th centuries celebrated Easter differently, it would either be on or before Nisan’s 14th day, which was the first day of Passover or on the Sunday on or before the first day of Passover. The reasons being that:

o Jesus’ passion, death and resurrection happened during Passover.
o All events prior to Easter is believed to have happened on the 14th day of Nisan.

325 AD
• Council of Nicaea decreed that there must be uniformity in observing dates for Easter and that it mustn’t overlap with the Jewish Passover. The reasons being that:
o Early Churches wanted to be disassociated with Judaism.
o The Jewish calendar confused the heads of the said Early Churches for Passover is also a movable feast.

• Through calculations of early churches’ calendars, the day that they believed to be the resurrection of Jesus was during the first vernal full moon which is spring.

• And this isn’t always the case ,where possibility of the 14th day Nisan to land on a day that wasn’t a first vernal full moon

• So to avoid the reliance on the Passover, the early church agreed that Easter is going to be celebrated on the Sunday after the first vernal full moon, the day right after the Jewish Passover.
Currently, the church uses the Gregorian Computus, a Gregorian computation for Easter dates, which is clearly imperfect considering the fact that Easter has on multiple occasions overlapped with the Passover. There has been 8 occurrences of Easter that has overlapped with the Passover, the next one being the Easter Sunday of 2183.

Source: rappler.com