// HACKER NEWS โ CYBERSECURITY
Show HN: How to mentally calculate the day of the week for any date
Some people can hear any date and name its weekday in about two seconds.
No calendar. No counting from today.
Not memorization, not magic: two secrets + a little practice.
By the bottom of this page, this is you. (Suspicious? Check those dates against a real calendar.)
Weekdays aren't a list โ they're a circle of 7.
You know remainders from school; here's what they're for.
Three challenges โ answer before the clock moves.
๐ Look at the table. Jumps of 7, 70, even 365 daysโฆ what actually decides where you land?
All of , with a squad of dates already marked in gold.
They share one spooky property โ spot-check any of them by tapping the calendar yourself.
The trick = nearest squad date โ leftover โ walk from the Doomsday. One together first.
One more fact โ you already own it: 365 รท 7 leaves remainder 1.
So each New Year the Doomsday slides +1 โ and +2 when the new year has a Feb 29:
Slides are +1 per year, +1 extra per leap โ but stepping 26 times is slow.
Why dozens? 12 slides + 3 leap extras = 15 โ and 15 on a clock of 7 is just +1.
One dozen = one slide.
For 19-something years the same steps work โ just start from
Wednesday, 1900's Doomsday, instead of Tuesday.
Time to train like a stage wizard. Three rounds โ the helpers disappear as you level up.