What day was I born on

The weekday you were born on, worked out from the calendar rather than looked up — along with the day of the year, the ISO week and whether it was a leap year.

Weekdays repeat on a 400-year cycle in the Gregorian calendar, which means the calendar for 2026 is identical to the one for 1626 and will be identical again in 2426. That cycle is why your birthday falls on the same weekday at intervals of 6, 11, 11 and 6 years — the pattern shifts by one day a year and two across a leap day, and works out to those gaps. One historical caveat: dates before the Gregorian changeover are calculated as if the current calendar had always applied, which is the proleptic convention. Britain and its colonies switched in 1752 and lost eleven days doing it, so a genuine record from before then may name a different weekday than this does.