March fourteenth is celebrated as Pi Day because the written date looks like the first digits of pi.

The longer version

Writing the date as three slash fourteen turns the calendar into the opening digits of pi, three point one four. Enthusiasts push the joke further and mark the moment at one fifty-nine, which appends two more digits, since the time and the date together read as three point one four one five nine.

The whole trick depends on the month-first convention. Places that write the day first cannot stage it in March at all, which is why twenty-two sevenths, the classic fractional stand-in, gets celebrated on the twenty-second of July and reads correctly in either order. Either way the holiday is a mnemonic, not a discovery.

Where this entry sits

This section collects odd corners of mathematics, from stubborn prime numbers to counting systems older than modern schools. Expect famous constants, probability puzzles, shapes that break everyday intuition, and numbers far too large to picture. The full list sits on the Numbers and Math page, where every entry is listed in order.