The Perpetual Calendar -- All You Really Need, Perhaps

The first day of the year can fall on only one of the seven days of the week. This year, 2026, started on a Thursday. Last year, 2025, started on a Wednesday; 2027 will start on a Friday. 2025, Wednesday; 2026, Thursday; 2027 Friday. So why bother buying a new calendar every year? Some people don’t; they use a Perpetual Calendar ¹ instead.

If there were only 364 days in a year, we could keep using the same calendar each year. The number 364 is evenly divisible by 7; there are exactly 52 weeks in that hypothetical year. But that one extra day in our year forces us to have a calendar for each day of the week.

What about leap years? They occur once every four years and have 366 days. We have to “skip over” a calendar every four years. For example, the calendar for 2028 (a leap year) will start on a Saturday. The following year will start not on Sunday but on a Monday.

Thus we need fourteen unique calendars; seven have 365 days; seven have 366, owing to the addition of February 29. This is what some refer to as “The Perpetual Calendar.”

Unfortunately, if you want to keep track of religious holidays, you’ll have to pencil in all those holidays whose schedules follow the lunar calendar ². And this involves more than figuring out the date of the Lunar New Year (aka Chinese New Year). Dates of holidays in the Abrahamic religions ³ (sometimes referred to as Judeo-Christian religions ⁴) also are based on the lunar calendar. This includes Christianity ⁵.

So is the Perpetual Calendar the only calendar you’ll ever need? That’s for you to decide.

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¹ https://en.wikipedia.org/wiki/Perpetual_calendar
² https://en.wikipedia.org/wiki/Lunar_calendar
³ https://en.wikipedia.org/wiki/Abrahamic_religions
⁴ The term “Judeo-Christian” ignores Islam, so I looked for a more appropriate term. See https://www.draslamabdullah.com/post/the-judeo-christian-tradition-history-paradox-and-political-construction
⁵ Yes even Easter requires reference to the lunar calendar. It falls on the date of the first full moon after the Vernal Equinox, roughly speaking. See: https://en.wikipedia.org/wiki/Date_of_Easter