Then Easter Day is the first Sunday after the first ecclesiastical full moon on or after 21 March. (This definition uses "on or after 21 March" to avoid ambiguity with historic meaning of the word "after". In modern language, this phrase simply means "after 20 March". The definition of "on or after 21 March" is frequently incorrectly abbreviated to "after 21 March" in published and web-based articles, resulting in incorrect Easter dates.) The reason for this is that Easter must occur after the biblical festival of Passover, on the full moon, when Jesus was crucified,” said Professor Sacha Stern, head of the Hebrew and Jewish Studies department at University College London.

Stern, Sacha, Calendar and Community: A History of the Jewish Calendar Second Century BCE – Tenth Century CE, Oxford University Press, Oxford, 2001. As the crucifixion and resurrection of Jesus happened after Passover, some early Christians decided to celebrate it then – on the 14th of the month of Nisan (from the Assyrian and Hebrew calendars). This correlates with March or April in the Gregorian calendar (named after Pope Gregory XIII), which is what we use today. Ayto, John, ed. (2009). Oxford Dictionary of English Idioms (3rded.). OUP Oxford. p.123. ISBN 978-0-19-954378-6.

Schaff, Philip; Wace, Henry, eds. (1890), The Synodal Letter, Nicene and Post-Nicene Fathers: Second Series, vol.14, The Seven Ecumenical Councils, Grand Rapids, Michigan, U.S.: Eerdmans Pub Co., pp.112–114, ISBN 0-8028-8129-7, archived from the original on 28 September 2020 , retrieved 4 July 2020 The ratios of (mean solar) days per year and days per lunation change both because of intrinsic long-term variations in the orbits, and because the rotation of the Earth is slowing down due to tidal deceleration, so the Gregorian parameters become increasingly obsolete. Explanation of the Gregorian solar and lunar calendar, with improved procedures over the tabular method, by David Madore

What is Passover? Why is it celebrated? What you need to know about the Jewish holiday How did the Easter Bunny and eggs become symbols of the holiday? A calendar of the dates of Easter, for the 95 years 532–626, marble, in the Museum of Ravenna Cathedral, Italy. Five 19-year cycles are represented as concentric circles. Dates are given using the system of the Roman calendar, as well as the day of the lunar month. So the first allowable date of Easter is March 22 + d + 0, as Easter is to celebrate the Sunday after the ecclesiastical full moon; that is, if the full moon falls on Sunday 21 March, Easter is to be celebrated 7 days after, while if the full moon falls on Saturday 21 March, Easter is the following 22 March.A table from Sweden to find the date of Easter 1140–1671 according to the Julian calendar. Each column corresponds to a period of 28 years. Notice the runes used as arbitrary symbols. Chronological diagram of the date of Easter for 600 years, from the Gregorian calendar reform to the year 2200 (by Camille Flammarion, 1907). Label The frequency distribution for the date of Easter is ill-defined, because every 100 to 300 years the mapping from golden number to epact changes, and the long-term frequency distribution is only valid over a period of millions of years (see below), whereas the system will certainly not be used for that long. From the perspective of those who might wish to use the Gregorian Easter cycle as a calendar for the entire year, there are some flaws in the Gregorian lunar calendar [46] (although they have no effect on the paschal month and the date of Easter): Easter is only ever celebrated by all Christians on the relatively rare occasions when the two calendars align,” she added.