Day of the Week Finder

How Day of the Week Finder Works

Pick any date -- centuries in the past or decades into the future -- and this tool tells you exactly what day of the week it falls (or fell) on, along with how it relates to today, its position in the year, and its ISO week number.

Formula & Method

The calculator builds a JavaScript Date object from the year, month, and day you pick and reads its built-in day-of-week value directly from the local calendar -- the same proleptic Gregorian calendar used for all dates in JavaScript, applied consistently even for dates before the Gregorian calendar was actually adopted. Day of year counts the days elapsed since January 1st of that year (inclusive), and the ISO week number follows the ISO-8601 convention where week 1 is the week containing the year's first Thursday.

Worked Example

Enter July 4, 1776, and the tool returns Thursday -- the day the United States Declaration of Independence was adopted. 1776 is also flagged as a leap year, and July 4th falls on day 186 of that year.

Frequently Asked Questions

How far back or forward can I check a date?

The date picker itself has no hard-coded limit -- you can select virtually any year your browser's date input allows, past or future. Just keep in mind that for dates far before the 1500s, results reflect the proleptic Gregorian calendar (today's calendar projected backward), not whatever calendar was actually in local use at the time.

What does "ISO week number" mean?

It's the week number under the ISO-8601 standard, where weeks run Monday to Sunday and week 1 is defined as the week containing the year's first Thursday. This can mean the very first or last days of a year sometimes belong to a week number from the adjacent year.

How is a leap year determined?

A year is a leap year if it's divisible by 4, unless it's also divisible by 100 -- in which case it's only a leap year if it's also divisible by 400. This is the standard Gregorian leap year rule (so 2000 was a leap year, but 1900 was not).