The Calendar Cipher

I was scrolling one night and just got bored of it, so instead I went and learned something I'd always thought was a cool party trick: figuring out what day of the week any date falls on, all in your head.

It comes down to four small numbers you add up (one for the year, one for the month, one for the day, one for the century) and then divide by 7. The remainder tells you the day. Once you know the keys it takes a few seconds per date.

I made this one-page reference to explain it properly and worked through the math myself until it actually made sense, not just something I'd memorized. Small project, but a good reminder that the time I'd have spent doomscrolling can just as easily go toward learning something random and fun instead.