The Christian holiday of Easter celebrates the resurrection of Jesus Christ. The Bible says Jesus died on a Friday but rose again on the third day, a Sunday.

Those who agree that this is literally true acknowledge that the exact date of the death and resurrection are unknown. Of course, those who follow the Christian religion and also believe in the literal birth of Jesus don't know the specific date either. So why is it that Christmas, the celebration of Jesus' birth, is a fixed date on our calendar, but Easter seems to hop around the spring calendar like ... well, like an Easter bunny?

In the Bible, the birth of Jesus didn't occur around any specific Jewish holiday. His resurrection, however, occurred around the Jewish celebration of Passover. In the early centuries following Jesus' death, some Christians would celebrate his resurrection on Passover; some would celebrate a week later.

Council of Nicaea

shamrock Before Easter can happen, the vernal equinox must take place. (Photo: SaigeYves/Shutterstock)

It wasn't until 325 that the Council of Nicaea took up the debate about the proper date to celebrate Easter. The council was convened by the Roman Emperor Constantine to settle issues that were dividing the church, the most important being the divinity of Christ. The Nicene Crede, a statement of faith still widely accepted and recited by Christians, came out of that council.

What the council didn't accomplish, however, was to create a fixed date for Easter. Instead, they came up with a way to determine a date for Easter each year that has confused many people since.

According to Time and Date, the council decided that Easter would be the first Sunday after the first full moon (modernly known as the Paschal Full Moon) after the vernal equinox — the day in the first half of the year when both day and night are exactly 12 hours long. On our modern calendar, it's the official first day of spring, which can fall on March 19, 20 or 21.

Easter can never happen before the vernal equinox, but it can happen on almost any Sunday after it up until April 25. This year, Easter will fall on April 21, 2019. If that seems much later than usual, there's a reason for that. In the last 365 years, the date for Easter has only been later than April 21 about 14 times, according to WLKY.

The only time Easter isn't on the Sunday after the full moon is when it would fall on the same day as the Jewish celebration of Passover. In that situation, Easter is celebrated the following Sunday.

The date of Easter affects other holidays

Float in New Orleans parade The date of Mardi Gras is determined each year by the date of Easter. (Photo: GTS Productions/Shutterstock)

Because the date of Easter hops around, it effects the dates of these other holidays and celebrations:

  • Good Friday, the day that Christians honor the crucifixion of Jesus, always falls on the Friday before Easter.
  • Palm Sunday always falls exactly one week before Easter. It's the celebration of Jesus' triumphant entry into Jerusalem on a donkey. His followers laid down branches, often thought to be palm leaves, ahead of the donkey for it to walk on.
  • Ash Wednesday is the beginning of Lent, which is celebrated for 40 days before Easter, not counting Sundays. It always falls on a Wednesday. The 40 days symbolize the 40 days that Jesus spent in the desert, fasting. Many Christians fast from something during those 40 days, sometimes sweets or alcohol, and much more recently, from social media.
  • Mardi Gras, also known as Fat Tuesday, is the day before Ash Wednesday. It's not a traditional Christian holiday but rather a day to celebrate excess before the period of Lenten fasting. The Mardi Gras celebration in New Orleans — which now begins well before Fat Tuesday — is a perfect example of that celebration of excess.
  • Public school spring break is also often determined by when Easter falls. Many school calendars are still based around mainstream Christian holidays, although Christmas break is now called winter break and Easter break is now called spring break.

Robin Shreeves ( @rshreeves ) focuses on food from a family perspective from her home base in New Jersey.

Why does Easter move around on the calendar?
The Council of Nicaea decided that Easter falls on the first Sunday after the first full moon after the vernal equinox.