Pick any date in YYYY-MM-DD form and the calculator returns the weekday it falls on, plus the day of year, ISO week number, and ISO week-numbering year. The weekday name is the long English form (Monday through Sunday), formatted by date-fns from a date parsed in UTC. The answer never shifts based on your local time zone, which matters for dates near midnight where viewer-local and UTC interpretations can disagree. For the seven-day numbering rule used here, see the ISO week-number lookup.
Common use cases
- Picking a date for a meeting or event. Before locking in a kickoff or a wedding, check what day of the week the candidate dates land on. A Tuesday client demo and a Saturday family dinner imply different prep windows. Enter the date and read the weekday name to confirm before sending invites.
- Verifying a historical or archival date. Genealogy entries, news archives, and old contracts often cite a date without the weekday. Enter the YYYY-MM-DD value to recover the day of the week. Pair the answer with the date arithmetic tool when the source quotes "the Friday three weeks before."
- Confirming a birthday or anniversary plan. Future birthdays, anniversaries, and renewal dates all hit different weekdays each year. Check whether the next milestone falls on a workday before booking a venue, requesting time off, or scheduling a delivery. The calculator handles any year inside the supported range.
- Planning around a deadline that excludes weekends. A contract due "by April 30" reads differently when April 30 is a Saturday. Look up the weekday first, then if the deadline crosses a weekend use the business-day counter to see how many working days you actually have left.
- Quickly verifying a quote like "Tuesday, July 20, 1969". When a source asserts a famous date with the weekday attached, the calculator confirms or refutes the claim in one query. Enter 1969-07-20 and read the weekday to check whether the source got Apollo 11's landing day right.
How it works
The calculator parses the YYYY-MM-DD input as a plain calendar date with no time-of-day component, anchors it to UTC, and asks date-fns for the weekday name in long English form. It also returns the day-of-year ordinal, the ISO 8601 week number, and the ISO week-numbering year, since those values often travel with weekday lookups. The math runs entirely against the proleptic Gregorian calendar, which is the convention every modern date library uses for dates inside our supported range.
Worked examples
Today
Enter date 2026-04-27.
Result: The calculator returns Monday.
April 27, 2026 is a Monday in the Gregorian calendar. The same date in any time zone reads Monday at any hour, because the calculator never converts to local time.
A famous date
Enter date 1969-07-20.
Result: The calculator returns Sunday.
Apollo 11 landed on the Moon on Sunday, July 20, 1969 (UTC). Some western timezones had already rolled into Sunday evening when the lunar module touched down at 20:17 UTC.
Christmas 2026
Enter date 2026-12-25.
Result: The calculator returns Friday.
Christmas Day in 2026 falls on a Friday, which means a four-day weekend in countries that observe Boxing Day on December 26. The same date in 2025 was a Thursday and in 2027 will be a Saturday.
Leap day
Enter date 2024-02-29.
Result: The calculator returns Thursday.
February 29, 2024 was a Thursday. The next leap day, 2028-02-29, falls on a Tuesday. Leap days do not get any special treatment by the weekday math; they are just an extra calendar day.
Edge cases & gotchas
- ISO numbering versus US numbering. The calculator returns the long English weekday name, so the numbering convention does not affect the visible answer. Behind that, ISO 8601 numbers Monday as day 1 through Sunday as day 7, while the US convention numbers Sunday as day 1 through Saturday as day 7. If you wire the weekday into another system, confirm which numbering it expects before passing an integer.
- Pre-Gregorian dates use the proleptic calendar. For dates before 1582-10-15 (the Gregorian cutover), date-fns extends the Gregorian rules backward in time rather than using the historical Julian calendar. A date like 1500-01-01 returns Wednesday in this calculator, but contemporary records would have called it a Saturday under the then-active Julian system. Use the proleptic answer for arithmetic; check a historical source for the contemporary weekday.
- The math runs in UTC, not your local time. YYYY-MM-DD inputs have no time-of-day component, so the calculator pins them to UTC midnight before resolving the weekday. A user in Sydney and a user in Honolulu both read the same answer for the same input. The trade-off: a "today" lookup near local midnight may disagree with what the wall clock says by one day.
- Far-future and far-past dates. date-fns handles dates thousands of years out without overflow, and the weekday cycle is exact (the Gregorian calendar repeats every 400 years). The supported-year window on this site is currently 2024 through 2032 for SEO pages, but the calculator itself accepts any year you can express in YYYY-MM-DD form.
Frequently asked questions about Weekday Calculator
Which weekday convention does the calculator use?
The visible answer is the long English name (Monday, Tuesday, ... Sunday), so numbering does not come up in the output. Internally it uses date-fns, which exposes both ISO numbering (Monday = 1) and US numbering (Sunday = 0); see the edge-case note above if you plan to feed the answer into another system.
Does it handle dates before 1582?
Yes, but using the proleptic Gregorian calendar. That means Gregorian rules are projected backward in time, so the answer for a date like 1500-06-15 is what the weekday would be if the Gregorian calendar had been in force. Contemporary records used the Julian calendar and would give a different weekday for the same date.
Does my time zone affect the answer?
No. YYYY-MM-DD is a date-only value with no time-of-day component, and the calculator anchors it to UTC midnight. A user in any time zone gets the same weekday for the same input. This matches how date-only fields work in databases and spreadsheets.
How is the day-of-year value computed?
It is the 1-based ordinal of the date inside its calendar year: January 1 is 1, December 31 is 365 in non-leap years and 366 in leap years. February 29, when present, is day 60 and shifts every later day by one compared to a non-leap year.
Why does the ISO week sometimes belong to a different year?
ISO 8601 places the first week of the year on the week containing January 4. Late-December dates can therefore belong to ISO week 1 of the following year (the ISO week-numbering year), and early-January dates can belong to ISO week 52 or 53 of the previous one. The calculator surfaces both the week number and the ISO week-numbering year for that reason.
Is there a quick mental check for a weekday?
Zeller's congruence gives an exact answer with one modular-arithmetic step. The Doomsday rule (anchor day per year, then offset to the date) is faster for mental work. Both produce the same weekday this calculator returns, since both are derived from the Gregorian calendar.
Can I use this for a recurring weekly check, like every Monday?
Yes. Enter a known Monday, then add multiples of 7 days to find the next ones, or use the date-add tool linked above. Every Monday in a given year shares the same day-of-year offset modulo 7, which is sometimes useful for spreadsheet schedules.
Glossary
- Weekday
- The day-of-week label for a date. The calculator returns the long English form (Monday through Sunday). The cycle repeats every 7 days indefinitely.
- ISO 8601 weekday numbering
- The international standard numbers Monday as 1 through Sunday as 7. Most non-US software defaults to this convention, including date-fns' getISODay function.
- US weekday numbering
- The North American convention numbers Sunday as 0 (or 1) through Saturday as 6 (or 7). Excel's WEEKDAY function and date-fns' getDay both default to a Sunday-first numbering.
- Day of year
- The 1-based ordinal of a date inside its calendar year. Ranges from 1 (January 1) to 365 in non-leap years, 366 in leap years.
- Proleptic Gregorian calendar
- The Gregorian calendar projected backward in time before its 1582 introduction. Used by date-fns and most date libraries for dates earlier than the historical cutover.
- Zeller's congruence
- A modular-arithmetic formula that returns the weekday for any date in the Gregorian or Julian calendar. The calculator uses date-fns rather than Zeller directly, but both yield the same answer for Gregorian inputs.