Weekday Finder — What Day of the Week

Pick any date and instantly see what day of the week it falls on, along with the details that usually take a calendar lookup: its ISO weekday number, the day-of-year, the ISO week number, whether it is a weekend, and how many days from today it is. Useful for planning, historical questions, and date math — all computed locally in your browser.

How to use the Weekday Finder — What Day of the Week

Choose a date with the picker, or press Today to jump to the current date. The large heading shows the weekday and the full date; the line beneath it tells you how many days in the past or future that is relative to today. The cards below break out the useful numbers: the ISO weekday (1 = Monday through 7 = Sunday), the common Sunday-first index (0 = Sunday), the day of the year, the ISO week number, and whether the date is a weekend.

The date picker accepts any year, so you can look up historical dates or plan far into the future. All calculations use your local calendar date — no time-of-day or timezone conversion is applied, so the weekday shown is the one for that calendar day everywhere it is that date.

Everything runs in your browser with the built-in date engine, so results are instant and nothing is sent anywhere.

Weekdays, ISO numbering, and week numbers

Asking "what day of the week is this date?" has a precise answer thanks to the rules of the proleptic Gregorian calendar, the system virtually all software uses. Given a year, month, and day, the day of the week is fully determined — there is no ambiguity — which is why a computer can tell you that a date centuries away lands on a Tuesday. Under the hood this is the same arithmetic behind hand methods like Zeller's congruence or the "doomsday" algorithm, but the browser's date engine does it for you exactly.

Numbering the days is where conventions diverge. The widely used programming convention from C and JavaScript numbers days 0 through 6 starting on Sunday, so Sunday is 0 and Saturday is 6. The international standard ISO 8601 instead numbers days 1 through 7 starting on Monday, making Monday 1 and Sunday 7. Mixing these up is a classic source of off-by-one bugs in scheduling code, so this tool shows both explicitly. Which counts as the "weekend" also varies by culture, though Saturday and Sunday is the most common definition and the one used here.

Week numbering follows ISO 8601 too, and it has a subtle rule worth knowing: week 1 is the week containing the year's first Thursday, equivalently the week containing January 4th. A consequence is that the first few days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. That is why the ISO week number is paired with an ISO week-year that can differ from the calendar year. The day-of-year, by contrast, is simply the ordinal count from January 1st (day 1) to December 31st (day 365, or 366 in a leap year) — handy for things like "the 100th day of the year" or computing elapsed days within a single year.

Common use cases

  • Planning and scheduling. Check which weekday a future deadline, event, or birthday falls on.
  • Historical lookups. Find the day of the week for any past date.
  • Avoiding numbering bugs. See both the Sunday-first (0–6) and ISO Monday-first (1–7) day numbers.
  • Week-based reporting. Read the ISO week number and day-of-year for logs, payroll, or fiscal periods.

Frequently asked questions

Does the time of day or timezone matter?
No. The weekday is a property of the calendar date itself, so the tool uses the date you pick directly with no time or timezone conversion.

Why are there two weekday numbers?
Different systems number days differently. JavaScript and C use 0–6 starting Sunday; ISO 8601 uses 1–7 starting Monday. Both are shown so you can use whichever your code or standard expects.

How is the week number calculated?
It follows ISO 8601: week 1 is the week containing the first Thursday of the year (the week with January 4th). Early-January or late-December dates may therefore belong to an adjacent week-year.

Can I look up very old or far-future dates?
Yes. The picker accepts any year and uses the proleptic Gregorian calendar, the standard used by software, so results are consistent across the full range.