The Gregorian calendar repeats its weekday pattern on a 400-year cycle, so any valid date —
past or future — maps to exactly one day of the week.
A specific date (like 25 December) falls on a different weekday most years, since 365 is not evenly divisible by 7 — the pattern only repeats fully every 28 years under stable leap-year rules, or 400 years accounting for century exceptions.
This uses the Gregorian calendar throughout, including for dates before it was adopted in a given country — treat very old dates as a calendar projection, not a historical record of what was actually observed at the time.
Can this tell me what day I was born?
Yes — enter your date of birth and the calculator shows the weekday it fell on, calculated the same way as any other date.
Does this work for dates centuries in the past or future?
Yes, using the standard Gregorian calendar rules throughout. Historical dates before a country adopted the Gregorian calendar (commonly 1582-1923 depending on the country) will not match what was actually written on calendars at the time.
Why does the same date fall on a different weekday every year?
A common year has 365 days, which is 52 weeks plus 1 day — so each year the same date shifts forward one weekday, or two after a leap year, until the pattern eventually repeats.