How this works
Pick a date — anywhere from Roman times to a thousand years in the future — and the calculator shows the day of the week, the day of the year (1–365 or 366), and the ISO-8601 week number. Useful for finding the day you were born, what weekday a wedding will fall on, what day of the year today is, or which ISO week to enter on a timesheet. The day name renders in your selected locale (Monday / Lunes / Lundi / Montag / 月曜日).
The formula
ISO-8601 weeks always start on Monday and the week containing 4 January is always week 1. This means a date in early January or late December may belong to a week of the previous or next year — the algorithm handles that by anchoring on the Thursday rather than the Monday. Day-of-year accounts for leap years automatically because it counts elapsed days from 1 January.
Example calculation
- Date: 28 April 2026.
- Day name: Tuesday.
- Day of year: 118 (Jan = 31, Feb = 28, Mar = 31, Apr = 28 → 31 + 28 + 31 + 28 = 118).
- ISO week: W18 (the Thursday of that week, 30 April, sits in week 18 of 2026).
Frequently asked questions
Why is my early-January date in week 52 or 53 of the previous year?
ISO-8601 says the week containing the year's first Thursday is week 1. So if 1 January falls on a Friday, Saturday, or Sunday, the calendar week it belongs to actually started on the previous Monday — in the previous year — and stays in that previous year's week count. Same effect at the end of December: 30 December 2024 is a Monday, so its week begins in 2024 and ends 5 January 2025, but it's still week 1 of 2025 because the Thursday (2 January) falls in 2025.
Does the calculator handle dates before 1582?
Yes, but it uses the proleptic Gregorian calendar — the modern calendar projected backwards. Most of Europe switched from Julian to Gregorian in 1582 (later in some countries — Britain in 1752, Russia in 1918), so dates before that have two possible weekdays depending on which calendar you mean. We use Gregorian throughout for consistency. For historical research where the exact contemporary weekday matters (e.g. records from 1500s England), look up which calendar that record was kept in.
Why does the day name appear in my browser's language and not the page language?
It shouldn't — we explicitly pass the page locale to Intl.DateTimeFormat. If you see a mismatch (e.g. "Tuesday" on the Spanish page), it's likely because your browser doesn't have the relevant locale data installed and is falling back. Modern Chrome/Firefox/Safari ship with all five of our locales bundled, so this is rare in practice. If you do see it, check your OS language packs.