How to Mentally Calculate the Day of the Week for Any Date: Conway's Doomsday Rule
Screenshots haven’t been added yet. Visit the product to take a look.
An educational interactive guide teaching John Conway's Doomsday Rule to calculate the weekday of any date mentally. It explains the underlying concepts with practice exercises and a final certificate of mastery.
Ratings & Reviews
Write a reviewBe the first to share a thoughtful review.
Information
Website checks
Automated checks of the public website by Google Lighthouse 13.5.0. Scores can vary between runs.
Checked Oct 25, 2026
Test this website on PageSpeedWebsite checks refresh automatically each week.
- Performance
- 99 out of 100, Good
- Accessibility
- 90 out of 100, Good
- Best Practices
- 100 out of 100, Good
- SEO
- 100 out of 100, Good
Around the Web
It's pretty bad form to give the doomsday algorithm and not give credit to John H. Conway who invented it. https://en.wikipedia.org/wiki/Doomsday_rule
seanhunter · Imported from Show HN
Cool site! When I do this trick I prefer using Bob Goddard's 'first Sunday' method [0]. It minimises the number of operations you need to do and makes it so you only need to keep track of one running total. [0] https://firstsundaydoomsday.blogspot.com/2009/12/quick-start...
zeech · Imported from Show HN
>The trick = nearest squad date → leftover → walk from the Doomsday. When i do the 31/OCT, the solution show 10/10 as nearest squadday, i though 11/7 is the nearest ?
mrheosuper · Imported from Show HN
Awesome site and guide, I just don't understand why all dates have the month in front of the date. Isn't this very confusing to anyone outside US? I get why it defaulted to this format, but in this day and age - it's trivial to automatically adjust it to the user's localization / preferences.
santah · Imported from Show HN
The difficult part, which is glanced over, is calculating the number of leap years for the year "slide" in Part 5. A bit more guidance there would go a long way.
eps · Imported from Show HN