Let us start with what we know. Jesus rose on a Sunday. This was the first Sunday that followed the feast of Passover. According to the Jewish calendar - Passover is a springtime festival and is observed on the 15th day of the Jewish Month of Nisan. The Jewish calendar is a lunar calendar and is based on the phases of the moon. The 15th day of Nisan is always a Full moon.

To this day, the Jewish people follow the Hebrew calendar to determine the date for Passover.

The first springtime full moon isn’t always going to fall on a Thursday, which means Easter may or may not fall on a Sunday!

In the early days of Christianity, many Christians, who were formerly Jewish, depended on the Jewish calendar and faithfully celebrated Easter three days after the feast of Passover.

For example, this year the date of the first springtime moon is March 28. This means Easter would fall on a Wednesday!

The first Ecumenical council of the Catholic Church held in Nicaea in 325 A.D., addressed the question of the observance of Easter. The goal was to have Christians all over the world observing Easter on the same day. A secondary goal was to not depend on the Jewish lunar calendar to set the date of Easter. The council of Nicaea decided on the following :

• Easter must be celebrated by everyone across the world on the same Sunday;

• This Sunday must follow the fourteenth day of the paschal moon;

• That moon was to be accounted the paschal moon whose fourteenth day followed the spring equinox.

Passover is a springtime festival. In the northern hemisphere, the astronomical spring is observed on March 21. We know this day as the spring equinox. This is the day when the length of the day is approximately equal to the length of the night.

Before the emergence of computers and satellites, people had to calculate dates by hand based on visual observation. Dates for Easter thus required astronomical calculations to predict the first full moon. This was no simple feat.

To ensure the date for Easter is based on ecclesiastical principles and not on astronomical observations, the church took the approach of following the Ecclesiastical moon or the phases of the moon that are calculated based on the ecclesiastical tables. These are calculations that predict the date of the first springtime full moon. These dates generally match the actual full moon dates but are not required to do so. These full moons are called Paschal full moons or Ecclesiastical full moons.

In case you were wondering, this year's Easter is celebrated on April 4. The first full moon after March 21 was March 28. April 4 is the first Sunday after March 28.

There is a lot more history and math to the calculation of the date of Easter. Check out the references if you’d like to read more.