This year Easter is on April 1st.  Last year it was on April 16 and next year it will be on April 21st.  It can vary from as early as March 22nd and as late as April 24th (I’m only using the Western Church dates, not Eastern). Why does the date on which Easter celebrated move around so much? The answer is Kinda Bonkers

To understand, we need to start with the Jewish calendar, which is a lunar calendar, as opposed to the solar based Gregorian calendar we are used to. Jewish months follow the phases of the moon with each new month starting with a “new moon” and a full moon occuring on the 14th of each Jewish month. The Jewish calendar is only 354 days instead of 365/366 days and thus every 2-3 years has a leap month to stay somewhat in sync with the Gregorian solar calendar.

Passover, an eight-day Jewish celebration, begins on the 14th day of the month of Nissan. Nissan jumps around a bit given the above discussion of lunar months and leap months, but by staring on the 14th day of Nissan means that Passover starts on the first full moon of spring.  What does this have to do with Easter?  The bible says that the Last Supper was held during Passover (a Thursday), Jesus died the next day (Good Friday) and then rose on Easter, three days later, on a Sunday.

In order to have the Last Supper, Good Friday and Easter occur within Passover, in 325 AD the Council of Nicaea declared that Easter is to be celebrated on the Sunday on or after the vernal equinox (which is the first day of spring). However, if that first full moon falls on a Sunday then both Passover and Easter are delayed a week. So, the first full moon can occur on the vernal equinox, or if it occurred just before the equinox it would next occur a month later. That’s why Easter jumps around so much.

To throw a little wrinkle into all this, in 325 AD the vernal equinox occurred on March 21st, so that is the date that is used for these calculations, even though the vernal equinox now usually occurs on March 20th.

Note – the Council of Nicaea was a conference called by the Holy Roman Emperor Constantine and included over 300 bishops in 325 AD that addressed many issues in early Christianity such as whether Jesus was eternal (i.e. did he exist before God created Him), affirmation of the Holy Trinity, and other important matters in addition to defining how the date for Easter would be determined.