What day was I born
Any date maps to exactly one weekday. 15 June 1990 was a Friday. The Gregorian calendar repeats on a 400-year cycle, so the same date in 2390 will fall on a Friday too.
How to find the day you were born
The 400-year cycle exists because the Gregorian leap rule: every four years, except centuries not divisible by 400 — produces exactly 146,097 days, and 146,097 divides by seven to give 20,871 whole weeks. That is not something the calendar was designed for; it is a happy accident of the arithmetic, and it means the entire pattern of dates and weekdays repeats identically every four centuries. One consequence is that weekdays are not quite evenly spread across dates. Over one full cycle the 13th of the month falls 4,800 times, and Friday takes 688 of them: more than any other weekday, and more than the 686 an even split would give. Friday the 13th is, very slightly, the commonest thirteenth there is. Within a single year the pattern is tighter still: 4 April, 6 June, 8 August, 10 October, 12 December, 5 September, 9 May, 7 November, 11 July and the last day of February always share a weekday. Learn that one weekday for a year and the whole calendar becomes reachable in your head.
Questions
Enter your date of birth and it is shown at the top.
Yes, exactly every 400 years: 146,097 days, which is 20,871 whole weeks.
Yes, marginally. Across a 400-year cycle it happens 688 times against 684 for the rarest weekday. The effect is real and tiny.
Yes. In any year, 4/4, 6/6, 8/8, 10/10, 12/12, 9/5, 5/9, 11/7, 7/11 and the last day of February all fall on the same weekday. Find that one day and count from the nearest anchor.
For Gregorian dates, yes. Before the switch from the Julian calendar the answer depends on the country and the year it adopted it. Britain lost eleven days doing so in 1752.
It is shown by birth month. Lists vary slightly between traditions.
The count from 1 January, from 1 to 365 or 366.