Easter 2015: When is it and why does the date change every year?

It's all down to the solar and lunar calendars

The date of Easter fluctuates depending on the calendars (Rex images)

This year Easter Day falls on 5 April but the date can vary by more than a month.

It’s all about our calendars

The question of Easter comes down to a matter of calendars: specifically, the Egyptian and the Hebrew.

The Egyptian version was based on the movement of the sun, which they passed on through Roman and then Christian cultures to become the modern standard. However, the Jewish calendar is based on the phases of the moon (as is the Muslim calendar).

Right. So, why does this affect when I get to eat chocolate?

It’s important because Easter is an attempt to bring together the solar (adopted by Christianity) and lunar (retained by the Jewish faith).

According to the Bible, Jesus was crucified on a Friday, known as Good Friday, but rose from the dead on the third day, which was a Sunday, and when Christians decided to celebrate with a feast day.

Their celebrations crossed over with the Jewish festival of Passover, which was fixed by the first full moon following the vernal equinox, the spring date when day and night are of equal length.

Problems occur because – very briefly – the lunar and solar calendars are different lengths and hugely complicated to calculate together accurately.

Who’s tried?

A bunch of people, but the one we care about is the decision made by the First Council of Nicea, which convened in AD325.

Nicea? Something from the Da Vinci Code ringing a bell here.

Take Dan Brown out of it.

This was a group of Christian bishops, gathered together in Nicea by the Roman Emperor Constantine, who thrashed out some of what we now assume to be the basic pillars of Christianity – including the date of Easter and the thorny question regarding the nature of the relationship between the Son of God and his father.

The bishops decided that Easter would be held on the Sunday after the first full moon of March, on or after the vernal equinox, which they fixed on 21 March. Should the equinox fall on the Sunday then Easter is delayed by a full week to the following Sunday.

So that was that, then? No more problems.

Unfortunately, the calculations were ever so slightly out – the vernal equinox now occurs on 20 March – and the established timetable does not perfectly align with the actual astronomical full moon.

Also many Christians, including Rome until the 6 century, actually ignored the ruling for centuries.

However, thanks in part to the decisions of these men, Easter can fall on any date between 22 March and 25 April for the Western Christian church (more on that later) and can occur before Passover.

This is an incredibly rare occurrence: the last time it happened was in 1818 and the next time it will occur is 2285.

Should the full moon occur on March 20, then the next full moon will be 18 April. This triggers a special rule and Easter is celebrated the following Sunday on 25 April.

OK, got that. Anything else?

Well, yes, actually. All of the above is only relevant if you are part of the Western Christianity. Eastern Christian beliefs follow different dates. They follow the dates laid down by Julius Caesar (called the Julian calendar) and is roughly 11 days behind our own – Gregorian – dates.