Why Easter is a movable festival?

If Easter is a reflection of the anniversary of the resurrection of Jesus, why does it have different dates in different years? Especially that its traditional anniversary of birth, or Christmas, is celebrated with a fixed date?

This is due to the traditional "date" of Jesus' death in the Bible. Well, it was on Friday, three days before the Jewish holiday of Passover. We mark this day in the calendar as Good Friday and in some countries, eg in Germany, it is also a day off from work.

The problem is that the Passover feast was celebrated during the "first spring full moon." What can this mean? Well, different days - depending on what phase the Moon is on the first calendar day of spring.

It is from the postponement of dates from the Jewish calendar to the astronomical lunar dates that we are celebrating Easter. In turn, the date of the birth of Jesus is defined in the Bible "eight days before the January calendar", which is a reference to a specific (non-moving) date.

How was the date of the Easter holiday counted?

In 325 CE, the Council of Nicaea determined that the date should be independent of external sources (including the Jewish calendar) and simply calculated. It is not known how calculations were made at that time, but tables were established in which dates for the following years were given.

In the following centuries many, as we would say today, date counting algorithms. They were called computuses . Computus were complicated because they had to take into account many factors such as calendar changes or leap years. Possible weekly dates of Easter are seven, but they do not repeat every seven years, but every 28 years (the so-called Easter solar cycle) - due to the presence of leap years. It's not about specific dates, it's about the numbers of weeks from the beginning of the year.

The algorithms of counting the Easter Night were later formed even more - including one proposed by the genius mathematician Carl Gauss in 1800. Interestingly, it was incorrect and Gauss corrected it several more times.

If we use Microsoft Excel, we are able to implement one of them as a cell formula. Assuming that in the A1 field, enter the year, in the next cell you can enter the following formula:

=ZAOKR.W.DÓŁ(DATA(A1 ; 5 ; DZIEŃ(MINUTA(A1/38)/2+56)) ; 7) - 34

Unfortunately, it calculates correctly the date of Easter only to 2078, but it is very simple:

The analysis of the data on the date of Easter shows that the Easter Sunday may occur on March 22 at the earliest, and on April 25 at the latest. However, not every Sunday in this "moon lottery" has equal opportunities. A simulation of 5.7 million years showed that the Easter Sunday usually falls on April 19.

Why Easter is a movable festival?