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.
How to use the what day was i born on
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. Inside that cycle a date returns to the same weekday four times every 28 years, at gaps of 5, 6, 6 and 11 years in whichever order your birth year lands in: the date shifts one weekday each common year and two across a leap day, and 28 years of that adds up to a whole number of weeks. The pattern only breaks at a century that skips its leap year, such as 2100, where the gap stretches to 12. 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.
Questions
Four times every 28 years, at gaps of 5, 6, 6 and 11 years. The order depends on where your birth year sits in the leap cycle.
Yes, every 400 years exactly. 2026 has the same calendar as 1626.
They are calculated proleptically, as if the Gregorian calendar had always applied. Britain lost eleven days switching, so historical records may differ.
A week numbering where week one contains the first Thursday of the year, so early January can fall in week 52 of the previous year.
Your device’s. The calculation runs in your browser, so it is today where you are rather than on a server somewhere else.