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THE DOOMSDAY RULE

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THE DOOMSDAY RULE

the weekday of any date in the Gregorian calendar, worked out in the head in three steps

Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.

Three sizes in one archive — from a sheet on the fridge to a poster on the wall.

Free

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Formats
A4 · A3 · A2
What you get
Print-ready PDFs, one per size, with every letterform converted to outlines — nothing to install, nothing to shift.
Subject
maths · everyone · memo · study

Read downwards: first the anchor date of the month, then the doomsday of the year — the anchor of the century plus the step of the year — and finally the difference between the date wanted and the anchor, taken modulo seven. The rule holds for the Gregorian calendar, which different countries adopted in different years from 1582 onwards.

THE 13TH FALLS ON A FRIDAY MORE OFTEN THAN ON ANY OTHER DAY

The Gregorian calendar repeats exactly every 400 years: that stretch holds 146 097 days, which is 20 871 whole weeks with nothing left over. So counting all 4 800 thirteenths of one such cycle settles the question for good.

They do not divide evenly: 4 800 is not a multiple of seven. The 13th falls on Friday 688 times and on Thursday 684. Friday the 13th is commoner than any other day, and that is a property of the calendar itself.

On the sheet

  • Read downwards: first the anchor date of the month, then the doomsday of the year — the anchor of the century plus the step of the year — and finally the difference between the date wanted and the anchor, taken modulo seven. The rule holds for the Gregorian calendar, which different countries adopted in different years from 1582 onwards.
  • Colour carries meaning, but meaning never rests on colour alone — the sheet reads in black and white.
  • Nothing about how it was made: the sheet carries only what the reader needs.

Printing

  • Three sizes in the archive: A4 — 210×297 mm, A3 — 297×420 mm, A2 — 420×594 mm.
  • All text is converted to outlines. No fonts to install, and nothing shifts on another machine.
  • White background and sparing fills — it prints on a home printer without draining the cartridge.
  • Margins of at least 10 mm: a home printer cannot print to the edge.
  • No stroke thinner than 0.3 mm, so nothing disappears in print.

Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 8 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.

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