Day of the Week Calculator

Find the day of the week for any date — past, present, or future. Also shows the day number of the year, ISO week number, and whether the year is a leap year. Includes a birthday feature that tells you what day of the week you were born on.

Where this gets used: a genealogist figuring out what weekday a great-grandparent was born in 1907 (which can confirm or falsify a handwritten family record); a trivia host building a round where contestants have to identify the day-of-the-week for historical events like the moon landing (Sunday, July 20, 1969); a contract writer setting a signing date for "the first Tuesday of next November" and needing to pick the exact calendar date. None of these are frequent problems, but when they come up, the wrong answer is embarrassing and annoying to verify otherwise.

Under the hood, recent dates (1970–present and near future) use the native JavaScript Date.getDay() method, which returns 0–6 for Sunday–Saturday and handles the Gregorian calendar including leap-year rules. Historical dates before 1970 still work because the Date object supports dates thousands of years in both directions — it models the proleptic Gregorian calendar, meaning the Gregorian rules are applied uniformly even to pre-1582 dates that were historically Julian. For most purposes this is what people want (a consistent mathematical calendar); for historical research that requires the actual Julian date, subtract the Julian-Gregorian offset for that century (~10 days in the 1500s, growing to ~13 days in the 1900s).

The ISO week number follows ISO 8601: week 1 of a year is the week containing the first Thursday of January, which means January 1 can fall in week 52 or 53 of the previous year. Leap-year detection uses the full rule: divisible by 4, but not 100, except also by 400 (so 2000 is a leap year, 1900 is not). Compared to typing into Google (which returns the correct day but not the year-day or ISO week), this tool surfaces all four facts at once. See also the option to calculate age from a birthdate and the calculate future dates tool for arithmetic on those dates.