First Sunday Doomsday Algorithm

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

Tuesday, October 11, 2022

Full-year Perpetual Cup

Here's a full-year perpetual calendar cup. Just one moving part, and you adjust it just once a year. All the months end properly at 30 or 31 days (except February). You can really plan ahead with this device!

Watch the video!

Wednesday, September 15, 2021

Calendar Cup

Here is a 200-year calendar in the form of a handy pencil cup.  I made it from PVC pipe, with laser-printed "sticker paper", and laser-printed plastic "waterproof paper" for the rotating sleeve. To lock in the setting, the floor of the cup is held, by a wing nut, to the inverted "floor" of a small base cup, which is glued to the sleeve. These floor disks are 3mm acrylic, laser-cut to match the pipe diameter.

Features of the cup:

  • Displays any month 1900 - 2099
  • Displays month name
  • Holds setting even when bumped or handled
  • Unambiguous instructions (align year with month)
  • Easy to find year: 00-99 in ascending order
  • Easy to find month: 12 red months in window
  • Rotation limited to valid angles
  • It's a useful desktop accessory!

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 1/23/2023]
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.