Weekday

This is a step-by-step interactive guide to John Conway's Doomsday rule, a mental calculation technique for naming the day of the week of any date in history without a calendar, phone, or counting from today. It breaks the method into two core secrets—the Leftover Trick (using remainders after dividing by 7) and the Secret Squad (a set of memorable anchor dates)—plus the yearly slide and a dozens trick for jumping across years. The page is built around practice challenges, a time machine year slider, and progressively harder rounds that remove helpers as you level up, aiming to get you to ~2-second speeds. It's designed for curious minds, math enthusiasts, and stage performers who want to master a classic recreational math skill, and it even offers a printable Certificate of Calendar Wizardry.