The Day of the Week Calculator answers a question that's surprisingly fiddly to do in your head: what day of the week does a given date fall on? It works for any date — last century or next century — and bundles in the day of the year, the ISO week number, and how many days away the date is, so a single lookup gives you the full picture. It also runs the math in reverse: add or subtract days to find a future date, or check the weekday you were born on.
How the weekday math works
Rather than rely on a browser's date object, this calculator computes the weekday with Zeller's congruence on the raw year, month, and day integers. The formula shifts January and February to be months 13 and 14 of the previous year, then combines the day, a month term, the year-of-century, and the century into a single value modulo 7. The result maps cleanly onto Sunday-through-Saturday. Because it's pure integer arithmetic, it returns the correct weekday for dates thousands of years in the past or future — well outside the comfortable range of typical timestamp math. The classic mental-math alternative, John Conway's Doomsday rule, reaches the same answers using a memorable set of anchor dates.
Leap years, day-of-year, and ISO weeks
The day of the year is just the running total of days in the months before the current one, plus the day of the month — with one extra day added after February in a leap year. The Gregorian leap-year rule is: divisible by 4, except centuries, which must be divisible by 400. That's why 2000 was a leap year and 1900 wasn't. The ISO-8601 week number is its own small puzzle: weeks start on Monday and week 1 is the one containing the year's first Thursday, so a date in early January can legitimately belong to week 52 or 53 of the prior year. All three figures are computed arithmetically here so they stay consistent with the weekday.
Julian vs. Gregorian — the proleptic assumption
The Gregorian calendar was introduced in October 1582 and adopted piecemeal over the following centuries. This calculator applies the Gregorian rules to all dates — the so-called proleptic Gregorian calendar. For modern dates that's exactly what you want. For dates before 1582, the computed weekday is internally consistent but may differ from the Julian-calendar date people recorded at the time (the two calendars had drifted about ten days apart by 1582). The calculator flags pre-1583 dates with a note so the assumption is never hidden. To work with whole-date durations instead of weekdays, see the Date Duration Calculator; for clock-time math, the Hours Calculator is the companion tool.