Christmas is on December 25th. The Fourth of July is on the 4th. Thanksgiving falls on a different date each year but is always the 4th Thursday in November. But Easter and Passover can occur in March or April. How can that be and why?
How are the dates of Passover and Easter determined and how early or late can they occur?
The date of Passover is determined each year by two criteria specified in the Torah.
According to Exodus 34:18, "The festival of matzot shall be kept at the appointed time in the month of Aviv"(meaning Spring). The Vernal Equinox (first day of Spring) normally occurs on the civil date of March 21st. But the Jewish time of Spring (Aviv) can occur in both March or April. The first month of the year of the Hebrew Calendar is Nisan. The month of Nisan is determined or begins according to when the moon is waxing to become full after or during the Vernal Equinox. So the Jewish month of Nisan can also occur or begin during March or April according to astronomical and lunar cycles. In addition, the first day of Nisan is determined by the date of the new moon (Rosh Chodesh).
The second criteria in the Torah determining the date of Passover is found in Leviticus beginning at 23:5.
"In the first month (Nisan), on the fourteenth day of the month, at twilight, there shall be a passover offering to the Lord - and on the fifteenth day of that month the Lord's Feast of Unleavened Bread." According to this passage, Passover is determined each year by counting 14 days after the new moon. Remember, the first day of Nisan is what ever day the new moon occurs. If new moon occurs at any time between March 6th and 20th - then on the 21st of March the moon will still be waxing to full and March would be the month that Nisan occurs in. If this is not the case...then the next lunar month is chosen. In that scenario, Nisan would begin in April on the date of the next new moon. And so... the earliest Passover can occur is March 21st - and the latest occurrence is April 20th.
The date of Easter is much easier to determine. Easter occurs on the first Sunday after the first full moon - on or after the Vernal Equinox (first day of Spring) That is a bit confusing, so instead of going in reverse...let's go forward. First - you have the arrival of Spring on the Vernal Equinox...normally March 21st. Following that - check the date of the next full moon. The Sunday following this full moon will be Easter Sunday. The earliest possible occurrence of Easter is March 22nd. This takes place when the full moon and the Equinox both occur on Saturday March 21st. Then Easter is the next day, Sunday the 22nd. The latest possible occurrence of Easter is April 25th. This happens when the the full moon occurs on the day before the Equinox and the next full moon falls on Sunday April 18th. Then Easter is the next Sunday...the 25th.