Our web page “Easter and other rambling holidays” includes rambling – that is, rather full – explanation of how the rule evolved by which the key date rambles over a span of 35 days in this random-looking pattern.
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I would say perfect timing, having recently finished “The Sun In The Church” by James Heilbron. That diagram is a thing of wonder.
There’s actually a quite prominent pattern in the diagram: diagonal lines of dates running downwards and to the left, separated by 3 years and either 3 or 4 days e.g. 2020, 2023, 2026, 2029, 2032, 2035. This isn’t a coincidence, because 3 calendar years are quite close to 37 lunar months, so the Easter Full Moons fall on similar days every 3 years. There are similar, albeit shorter, diagonals running the other way at 8-year intervals, for the same reason.
Your diagram prompted me to update my own web site dateofeaster.com to include a section on patterns in the dates of Easter.
And yet, in your chart, there *is* a distinct pattern. There are clear diagonal runs of Easter year labels separated by three years, with a four day gap in the date of Easter: 2021-2024-2027, 2020-2023-2026-2029-2032-2035 and so on. Easter in year Y+3 is four days earlier than in year Y, unless Y is a leap year, when the gap is three days. This is no coincidence, because three calendar years are very close to 37 lunar months, so the Paschal Full Moon falls on (almost) the same day after a three-year gap. The three- or four-day difference in the date of Easter is then simply due to the calendar shifting by a day every year, or two days if a leap year is involved, moving Sundays by three or four days in the calendar.
There are also strong patterns in the date of Easter in years Y and Y+1, as the diagram at my own web site dateofeaster.com demonstrates.
It’s well and truly dawned here. We’re about commence Easter Sunday mass.