What day was it?

Field notes

Why does the weekday shift each year?

A common year is 52 weeks and one day, so your birthday moves forward one weekday per year — two after a leap day passes. That’s the whole mystery, and why the “same date next year” line rarely matches.

Where does this run?

Entirely in your browser. Dates you type are never sent anywhere — there is no server doing the math, no analytics watching you do it.