Find the day of the week of any date from 1583 to 9999, with day of the year, ISO week and leap-year facts.
Most people know the day of the week they were born only because someone told them. A perpetual calendar answers it for any date, and the Gregorian calendar makes that possible because it repeats: the pattern of weekdays and leap years closes a full cycle every 400 years, exactly 20,871 weeks. Any date can therefore be mapped to a weekday with pure arithmetic — no lookup table, no calendar to leaf through.
This tool converts the date to a Julian day number and reads the weekday from it, and also shows the result of Zeller's congruence, the classic 19th-century formula that gets to the same answer with a handful of divisions. Alongside the weekday it computes the day of the year, the days remaining until 31 December, the ISO 8601 week number, and whether the year is a leap year — a year divisible by 4, except centuries not divisible by 400, which is why 1900 was not a leap year and 2000 was.
The range starts in 1583 on purpose. The Gregorian calendar was introduced in October 1582, when ten days were simply deleted from the calendar — 4 October was followed by 15 October in the countries that adopted it first, while others waited centuries. Any weekday claim about earlier dates depends on which calendar and which country you mean, so the tool stays inside the range where a single answer is correct. The calculation happens entirely in your browser.
Zeller's congruence (Gregorian): h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ + 5J) mod 7, where q is the day, m the month with January and February counted as 13 and 14 of the previous year, K the year within the century and J the century. h = 0 is Saturday, 1 Sunday, up to 6 Friday. Leap year: divisible by 4, except centuries, unless divisible by 400.
Type your date of birth and the answer appears instantly. The tool works for any date from 1583 onwards, so it covers every living person and several generations before.
Because the Gregorian calendar only began in October 1582, and different countries adopted it at different times — England in 1752, Russia in 1918. Before that, a weekday depends on which calendar you mean, so a single answer would be misleading.
It is the ISO 8601 week number, the standard used in industry and logistics. Weeks start on Monday and week 1 is the one containing the first Thursday of the year.
A year is a leap year when it is divisible by 4, except years divisible by 100, unless they are also divisible by 400. So 1900 was not a leap year, but 2000 was.
No. The weekday is calculated in your browser with JavaScript and no data leaves the page.
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