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.
