Weekday Calculator
Find what day of the week any date falls on.
Enter a date and click Find Day of Week to see the results.
Disclaimer: The results provided by this tool are estimates for informational purposes only. Actual values may vary. Please verify important calculations independently.
How the weekday is determined
Finding the weekday for an arbitrary date reduces to a modular-arithmetic problem once you have a reference date of known weekday. The tool internally converts the target date to a day count relative to a known anchor, then takes that count modulo 7 to select the weekday. Because 365 leaves a remainder of 1 when divided by 7, each non-leap year shifts the weekday of a given date forward by one day; leap years shift by two for dates from March onward. That is why your birthday lands on a different day of the week each year in a repeating 5-6-11-6 pattern across typical four-year cycles.
The classic mental-math method for weekday calculation is Zeller's congruence, a formula published by Christian Zeller in 1882 that returns the weekday directly from year, month, and day without needing an anchor date. Modern calculators use lookup-table variants because they are faster, but the result is the same. The tool also reports the day-of-year and the ISO week number so you can see the target date positioned within multiple useful coordinate systems at once, which is helpful for historical research and for planning recurring events.
Real-world weekday lookups and planning
The most common question a weekday calculator answers is 'what day of the week was I born?' followed closely by 'what day of the week is my wedding date, and is that a weekend?' Historical research leans on weekday lookups for verifying dated documents — if a letter is dated Tuesday, October 14, 1947, you can confirm that October 14, 1947 actually was a Tuesday. Genealogists use the same check to catch transcription errors in birth, marriage, and death records. Legal practitioners verify filing dates against court business-day calendars, where the weekday determines whether a deadline falls on an observed holiday.
Forward-looking uses include planning events on preferred weekdays. A couple choosing between anniversaries one year apart can check which date falls on a Saturday. Project managers picking a go-live date often avoid Mondays (risky for weekend deployments) and Fridays (risky for weekend pages). Sports scheduling, tax filing deadlines, and academic calendars all have weekday conventions — US federal tax day shifts to the next business day if April 15 falls on a weekend or DC-observed holiday, for example. The weekday calculator is the fastest way to resolve those shifts without pulling up a full calendar.