Day of the Week Calculator

1. 01Pick a dateUse the date picker to choose any date between 1900 and 2200.
2. 02Read the day of the weekThe weekday appears immediately, along with the ISO week number, day of the year, quarter, and leap-year status.

• Finding what day a past date fell onCheck what day of the week you were born, or what day a historical or personal date fell on.
• Planning around a future dateFind out what day of the week a holiday, deadline, or event date will fall on.
• Getting the ISO week number for a dateLook up the ISO 8601 week number for scheduling, reporting, or systems that use week-based dates instead of calendar dates.

• January 1, 2000Falls on a Saturday — the well-known "Y2K" date.
• A late-December date in an ISO week edge caseDecember 31, 2007 (a Monday) belongs to ISO week-year 2008, week 1 — not week 53 of 2007 — because ISO week 1 is defined as the week containing that year's first Thursday, and that week starts on December 31, 2007.
• A leap dayFebruary 29, 2024 falls on a Thursday and is day 60 of that (leap) year.

This tool treats every date as a pure calendar date and calculates entirely in UTC at midnight, with no timezone or daylight-saving-time assumptions — the same date always produces the same weekday everywhere, regardless of where you are.

ISO weeks start on Monday, and week 1 of an ISO week-year is defined as the week containing that year's first Thursday (equivalently, the week containing January 4). Because of this, the last few days of December can belong to ISO week 1 of the following year, and the first few days of January can belong to the last week (52 or 53) of the previous year. This tool calculates the ISO week-year correctly for those boundary dates rather than assuming it always matches the calendar year.

A year is a leap year if it is divisible by 4, except that century years (divisible by 100) are not leap years unless they are also divisible by 400. This is why 2000 was a leap year but 1900 and 2100 are not.

Why is a "week year" sometimes different from the calendar year?

Because ISO 8601 defines week 1 of a year as the week containing that year's first Thursday, a date in late December can fall in week 1 of the following year, and a date in early January can fall in the last week of the previous year. This tool calculates that correctly rather than assuming the week-year always matches the calendar year.

Why is 1900 not a leap year, but 2000 is?

The Gregorian calendar treats century years as an exception: they are leap years only if divisible by 400. 1900 and 2100 are divisible by 100 but not 400, so they are not leap years; 2000 is divisible by 400, so it is.

What range of dates does this tool support?

Any calendar date between January 1, 1900 and December 31, 2200.

Is my data saved?

No. Everything is calculated in your browser; nothing is sent to a server.