Why does Jesus's death day change?

Answer 1

I'm not 100% certain what you mean by "Jesus' Death Day". If you mean "why does the date of Easter (and therefore of Good Friday) change from year to year", it's partially because Easter is always on a Sunday and therefore Good Friday is always on a Friday (as the name implies). However, there's a little more to it than that, because that explains a swing of at most 7 days, and Good Friday varies a lot more than that.

The Hebrews/Israelites/Judeans used a lunar-solar calendar to determine festival dates, so Jewish holidays (which use the traditional Hebrew calendar) tend to wander around a bit on the modern purely solar calendar. This is because there are between 12 and 13 lunar months per solar year, so the calendar either falls behind or runs ahead depending on whether you use 12 or 13 lunar months per lunar year. The Hebrew calendar is a mixture of both: some years have 12 months and some have 13, so some years it runs ahead and some years it lags behind, and it all averages out. Depending on whether it happens to be a run-ahead year or a lag-behind year and exactly how far out of sync the purely lunar calendar is with the solar one, the date of Easter can vary by up to 35 days.

Answer 2

Our Blessed Lord's death on the cross happened once for all in the year 33 A.D., that cannot change, it is part of history. If you are referring to the date when it is commemorated each year-Good Friday, that always falls two days before Easter Sunday. As Easter Sunday is determined each year by a complicated formula (next paragraph), it moves around a lot in our modern Gregorian calendar.

In A.D. 325 the Council of Nicaea established that Easter would be held on the first Sunday after the first full moon occurring on or after the vernal equinox. From that point forward, the Easter date depended on the ecclesiastical approximation of March 21 for the vernal equinox.

Easter is delayed by 1 week if the full moon is on Sunday, which decreases the chances of it falling on the same day as the Jewish Passover. The council’s ruling is contrary to the Quartodecimans, a group of Christians who celebrated Easter on the day of the full moon, 14 days into the month.