First Sunday Doomsday Algorithm

A method for finding the day of the week for any date -- in your head.

Thursday, May 3, 2018

New Perpetual Calendar

I set myself a goal: to make the simplest device that can find the day of the week for any date. I think you'll agree that a device made from a single sheet of paper is as simple as you can get.

Here is my Perpetual Calendar (click to view the PDF file). With just one moving part -- a sliding window you cut off the sheet itself -- it displays a month's calendar for any date in the Gregorian calendar.

All you need is scissors and access to a printer. It will only cost you one sheet of paper. No mental algorithm required! The expiration date for this device is: never!

Here is a non-USA version (still in English), which fits A4 paper, and starts the week on Monday instead of Sunday.


Here is an earlier design, the One-page 200-year calendar. Also a single sheet of paper, it is limited to the years 1900 - 2099. It folds neatly in thirds, like a letter.

Thursday, January 27, 2011

Learn by Example

[Revised 7/29/2011]
This quick summary will show you, by example, how to determine the day of the week for any date -- in your head! (See the longer post for the full story.)

Wednesday, December 2, 2009

Learn the First Sunday Doomsday Algorithm

[Revised 8/17/2011]
This longer article will show you how to determine the day of the week for any given date -- without consulting a calendar. It covers some history (of calendars and methods) and explains how this method works, and why it works.