Calendar · Days · Weekdays
Day of the Week
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Find what day of the week any date falls on — past or future. Includes a full month mini-calendar, date properties, historical famous dates, and a weekday finder for any month.
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📅 Find All [Weekday]s in a Month
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🏛️ Famous Historical Dates
How to Find the Day of the Week
Every date in history corresponds to exactly one day of the week. The Gregorian calendar repeats in a 400-year cycle with exactly 97 leap years, giving a total of 146,097 days — divisible by 7, so the cycle repeats exactly every 400 years.
The Doomsday Algorithm (Mental Math)
John Conway's Doomsday algorithm lets you calculate the day mentally:
1. Certain dates always fall on the same day of the week in any given year —
called "Doomsday" dates: 4/4, 6/6, 8/8, 10/10, 12/12, last day of Feb,
5/9, 9/5, 7/11, 11/7 ("I work 9-5 at 7-11")
2. Find the Doomsday for the year:
Century anchor: 1800→Fri, 1900→Wed, 2000→Tue, 2100→Sun
Year adjustment: a = year%100÷12, b = year%100 mod 12,
c = b÷4, Doomsday = (anchor + a + b + c) mod 7
3. Count from nearest Doomsday date to your target date
What day of the week repeats every year?
In a non-leap year, the calendar advances by 1 day per year. So if January 1 was Monday this year, it'll be Tuesday next year (unless next year is a leap year, in which case it advances by 2 days). Every date shifts forward 1–2 days per year. To find the exact same calendar (same dates on same weekdays), you have to wait 6, 11, or 28 years depending on where you are in the leap year cycle. The full calendar repeats every 28 years (in non-century years).
What is ISO week numbering?
ISO 8601 defines week numbers from 1 to 52 (or 53). Week 1 is the week containing the first Thursday of the year (equivalent to the week containing January 4). Weeks run Monday to Sunday. This means January 1 might be in week 52 or 53 of the previous year, and December 31 might be in week 1 of the next year. ISO weeks are widely used in business, shipping, and international scheduling.
How many of each weekday are in a year?
In a 365-day year, each weekday appears either 52 or 53 times. Exactly 1 day of the week appears 53 times (the day January 1 falls on, and the next day in a leap year). The other 6 days appear 52 times. So if January 1 is a Monday in a non-leap year: Monday appears 53 times, all others 52 times. Total: 365 = 52×7 + 1. In a leap year: 2 days appear 53 times each. Total: 366 = 52×7 + 2.