How this day of the week calculator works
This calculator answers questions like "what day of the week is 25 December 2026?" or "what day was I born?". You give it a date and it works out the weekday — Sunday through Saturday — on the standard Gregorian calendar. To keep the answer accurate it builds the date at noon UTC rather than local midnight, which avoids the off-by-one error that can shift a weekday when a browser sits behind or ahead of UTC.
Alongside the weekday it reports the day-of-year number (1 January is day 1), the ISO 8601 week number (weeks start on Monday and week 1 is the week containing the first Thursday of the year), whether the date is a weekend, and how many days from today it is. The underlying math could equally be done with Zeller's congruence or the Doomsday rule — pen-and-paper formulas that find the weekday from the day, month and year — but the result is identical.
| Date | Day of the week | Day of year |
|---|---|---|
| 1 Jan 2000 | Saturday | 1 |
| 2 Jun 1990 | Saturday | 153 |
| 25 Dec 2026 | Friday | 359 |
| 29 Feb 2024 | Thursday | 60 |
Reference note: calculations use the proleptic Gregorian calendar and noon UTC for the chosen date, so weekdays are stable regardless of your time zone. Leap years are handled automatically. Weekdays for dates before a country's Gregorian adoption are proleptic.
Frequently asked questions
- What day of the week was or is a given date?
- Enter any date and the calculator tells you which weekday it falls on — for example Tuesday or Saturday — using the Gregorian calendar. It works for past dates, today and future dates alike.
- What day of the week was I born?
- Type your date of birth into the date box and the tool shows the weekday you were born on. For example, 2 June 1990 was a Saturday. This is one of the most common reasons people look up the day of the week for a date.
- How is the weekday calculated?
- The weekday is derived from the date using the proleptic Gregorian calendar. We build the date at noon UTC to avoid time-zone off-by-one errors, then read its weekday index, which maps 0 to Sunday through 6 to Saturday.
- What is Zeller's congruence?
- Zeller's congruence is a classic formula that computes the day of the week for any date using only arithmetic on the day, month and year. The Doomsday rule is a related mental-math shortcut. This tool uses reliable built-in date math, but those algorithms give the same answer.
- Does it work for any year?
- Yes. It works across a very wide range of past and future years, including leap years, which are handled automatically. Weekdays before each country's 1582–1923 Gregorian switchover are proleptic.
- What is the day-of-year number?
- The day-of-year number counts how many days have elapsed since the start of that year, where 1 January is day 1. For example, 31 December is day 365 in a common year and day 366 in a leap year. The tool also shows the ISO week number.