Day of the Week Calculator
Find the weekday name for any calendar date, past or future.
Pick a date to find its weekday.
To find the day of the week, read the date at UTC midnight and take its weekday index (Sunday = 0 … Saturday = 6). For example, 15 August 2026 falls on a Saturday — day 227 of the year, in ISO week 33. The tool works for any past or future date.
How the weekday is worked out
The weekday of any date follows a fixed 7-day cycle, so it can be computed exactly rather than looked up. This tool reads your date at UTC midnight — free of time-zone or daylight-saving shifts — and takes its weekday index, where Sunday = 0 through Saturday = 6, then maps that index to a name. Alongside the weekday it reports the day of the year (1–366), the ISO-8601 week number, whether the date is a weekday or a weekend, and how many days separate it from today.
Zeller’s congruence (Gregorian): h gives the weekday where 0 = Saturday … 6 = Friday. q = day, m = month (March = 3 … February = 14, so January and February count as months 13 and 14 of the previous year), K = year mod 100, J = ⌊year ÷ 100⌋.
Worked example: 15 August 2026
August is month 8, so no January/February adjustment is needed and the year stays 2026.
- 1 Adjust January and February. Only January and February are renumbered as months 13 and 14 of the previous year. August is month 8, so m = 8 and the year stays 2026.
- 2 Set the pieces. q = 15, m = 8, K = 2026 mod 100 = 26, and J = ⌊2026 ÷ 100⌋ = 20.
- 3 Add the terms. 15 + ⌊13 × 9 ÷ 5⌋ + 26 + ⌊26 ÷ 4⌋ + ⌊20 ÷ 4⌋ − 2 × 20 = 15 + 23 + 26 + 6 + 5 − 40 = 35.
- 4 Take the result mod 7. 35 mod 7 = 0, and h = 0 means Saturday — so 15 August 2026 is a Saturday.
Weekday index reference
This tool numbers days with JavaScript’s getUTCDay (Sunday = 0). Zeller’s congruence uses a different origin (h = 0 is Saturday), so the two columns do not match — check which convention a source uses before comparing.
| getUTCDay index | Weekday | Zeller h |
|---|---|---|
| 0 | Sunday | 1 |
| 1 | Monday | 2 |
| 2 | Tuesday | 3 |
| 3 | Wednesday | 4 |
| 4 | Thursday | 5 |
| 5 | Friday | 6 |
| 6 | Saturday | 0 |
Calendar edge cases
It uses the proleptic Gregorian calendar. Weekdays are computed with the modern Gregorian rules extended backwards to every year, including dates before the calendar existed. That keeps the 7-day cycle consistent across the whole range, but it is a mathematical projection, not what people living then would have written.
Pre-1582 dates can differ from history books. Most of Europe used the Julian calendar until the 1582 reform (and some countries switched much later), when 10 days were dropped to realign the seasons. A history source quoting a Julian date will often name a different weekday than the proleptic Gregorian result shown here, so state which calendar you mean when the date is old.
Leap years shift the day of the year. After 29 February, a date sits one ordinal position later than in a common year, which is why the day-of-year figure runs to 365 in most years and 366 in leap years.