How the date of Easter is set

1. Easter and Holy Week

Easter is the most important festival in the Christian year. Christians believe that Jesus was crucified on a Friday and that God raised him from the dead on the Sunday: the resurrection. All four Gospels say that women came to the tomb early on the first day of the week, the Sunday, and found it empty. That is why Easter is always on a Sunday, and why Christians meet for worship on Sundays all year round.

The week before Easter is called Holy Week.

2. Why Easter follows the moon

The Gospels set the last week of Jesus’ life in Jerusalem at the time of the Jewish festival of Passover (Pesach), which remembers the escape of the Israelites from slavery in Egypt. Matthew, Mark and Luke describe the Last Supper as a Passover meal. Passover is set by the Jewish calendar, which follows the moon: it begins on the 15th day of the spring month of Nisan, at full moon. So from the start, Christians linked Easter to the spring full moon too. The Welsh word for Easter, y Pasg, comes from Pesach through the Greek and Latin word Pascha.

In the early centuries, different churches worked out the date in different ways. The Council of Nicaea, a meeting of bishops in the year 325, is traditionally linked with agreeing that all Christians should celebrate Easter on the same Sunday. A single method of working out that Sunday only gradually came into use over the centuries that followed.

3. The 19-year moon cycle

A full moon comes round about every 29.53 days, so full moons do not land on the same date every year. But every 19 years the phases of the moon fall on almost exactly the same dates again, because 235 moon cycles and 19 years both come to about 6,940 days. This is called the Metonic cycle, after Meton, an astronomer in ancient Athens. The church’s full moon tables are built on this 19-year cycle.

4. Two calendars, two Easters

In 1582 Pope Gregory XIII brought in a corrected calendar, the Gregorian calendar, which most of the world uses today. Britain, including Wales, changed over to it in 1752. Most Orthodox churches (for example in Greece, Romania and Russia) still work out Easter using the older Julian calendar, so Orthodox Easter is often 1, 4 or 5 weeks later than Western Easter. In some years the two fall on the same day.

5. Lent, Ascension and Pentecost

• Lent is 40 days of preparation before Easter, remembering the 40 days Jesus spent fasting in the wilderness. Many Christians give something up or take on something good. Lent begins on Ash Wednesday (Dydd Mercher y Lludw), which is 46 days before Easter Sunday: the 6 Sundays in Lent are not counted as fast days, and 40 + 6 = 46. The day before Ash Wednesday is Shrove Tuesday, or Pancake Day.
• Ascension Day is the 40th day of Easter, counting Easter Sunday as day 1. Christians remember Jesus returning to heaven.
• Pentecost (y Sulgwyn, Whitsun in English) is the 50th day counting Easter Sunday as day 1. That makes it 49 days after Easter Sunday, the seventh Sunday after Easter. Christians remember the Holy Spirit coming to the disciples (Acts, chapter 2).

6. Easter Sunday, 2020 to 2040

Use this table to check the answers your algorithm gives. Western Easter is the date used by the Church in Wales. Orthodox Easter is shown as a date in our (Gregorian) calendar.