Day of the Week Calculator
| Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|
29 | 30 | 31 | 1 | 2 | 3 | 4 |
5 | 6 | 7 | 8 | 9 | 10 | 11 |
12 | 13 | 14 | 15 | 16 | 17 | 18 |
19 | 20 | 21 | 22 | 23 | 24 | 25 |
26 | 27 | 28 | 29 | 30 | 1 | 2 |
Day of the week
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How to Use
Pick a date from the date picker. The tool instantly tells you the day of the week (Sunday through Saturday), the day-of-year number (1-366), the ISO 8601 week number, the ISO week-numbering year, and whether the year is a leap year. Useful for quickly answering questions like "what day was I born?" or "what day of the week was 9/11?".
How the Calculation Works
The day-of-week calculation uses Zeller's congruence, a closed-form formula that maps any Gregorian date directly to a weekday index without iterating day by day. Day-of-year sums the lengths of months before the given month, then adds the day. ISO week numbers follow the ISO 8601 standard: weeks start on Monday, and week 1 is the week containing the first Thursday of the year — which is why early-January dates can fall in the previous year's week 52 or 53.
About ISO Weeks
The ISO 8601 week-numbering year sometimes differs from the Gregorian year for dates near year boundaries. For example, January 1st of a year that starts on Friday belongs to ISO week 53 of the previous year. The displayed "ISO year" reflects this: week 53 of 2026 might be labeled with ISO year 2026 even though the Gregorian date is in 2027. This is the standard used by most European countries and many business calendars.
Common Use Cases
- Answering "what day was I born?" or other historical date trivia.
- Planning events: which weekday will a date fall on next year?
- Quick verification when filling out forms that require the weekday.
- Checking ISO week numbers for sprint planning or fiscal-week reporting.
- Confirming that a year is a leap year before computing date offsets in scripts.
Tips
- Leap years occur every 4 years, except for years divisible by 100 (unless also divisible by 400). 2000 was a leap year; 1900 was not.
- ISO weeks always start on Monday — week 1 is the week containing the first Thursday of the year.
- Day of year goes 1 through 365 (or 366 in leap years).
- This tool uses the Gregorian calendar (the modern civil calendar). For dates before 1582-10-15, results would not match the Julian calendar in use at the time.
- The calculation is deterministic and time-zone-independent: the result depends only on year, month, and day.
Privacy
All calculations happen in your browser. The dates you enter are never sent to any server or stored anywhere.