Weekday Finder

Paste a list of dates, one per line, and get each one's weekday, day of year and ISO week back as a table. The weekday name is rendered by the browser's own locale data, so you can read the same list in any of the supported languages, and the whole table copies out as tab-separated text that drops straight into a spreadsheet. Batch is the point: checking one date is quick anywhere, checking the launch dates of forty releases is not.

Dates are interpreted in UTC rather than your local zone, which is deliberate and occasionally surprising. A bare 2026-09-17 has no time attached, so the only consistent reading is midnight UTC; if the dates came out of a system that logged them in local time, one near midnight can name the neighbouring day. The strict YYYY-MM-DD form is parsed directly, and anything else falls through to the browser's own date parsing, which is lenient and inconsistent between engines — if a line matters, write it in ISO form.

The ISO week column carries its own week-year, because that is the only way to make it true. The first ISO week of a year is the one containing its first Thursday, so 1 January 2023 belongs to week 52 of 2022, and 31 December 2024 belongs to week 1 of 2025. Reading a week number without the year it belongs to is how a report loses a week every few years.

There is an odd fact underneath all of this worth knowing if you ever need to sanity-check a calendar. The Gregorian calendar repeats exactly every 400 years — 146,097 days, which is precisely 20,871 weeks with nothing left over — so any date's weekday is the same as it was 400 years earlier, and the pattern never drifts. That also settles a piece of folklore: across a full 400-year cycle the 13th of the month falls on a Friday 688 times out of 4,800, more often than on any other day of the week. Friday the 13th really is the most common combination, by a margin of four occurrences over Thursday and Saturday.