Day of the Week Calculator
Enter any date to find out which day of the week it falls on, along with the ISO week number.
The Gregorian Calendar
The Gregorian calendar, introduced by Pope Gregory XIII in 1582, is the calendar system used worldwide today. It divides the year into 12 months with 365 days, adding a leap day (February 29) in years divisible by 4 — except for century years, which are only leap years if divisible by 400. This means 2000 was a leap year, but 1900 was not.
Because the calendar repeats every 400 years (a span of exactly 97 leap years and 303 regular years), any date pattern recurs on a 400-year cycle. The total number of days in 400 years is exactly 146,097, which is divisible by 7 — meaning the day-of-week pattern repeats perfectly every 400 years.
Notable Dates & Their Days
| Date | Day of Week | Event |
|---|---|---|
| Jul 4, 1776 | Thursday | US Independence Day declared |
| Nov 11, 1918 | Monday | WWI Armistice signed |
| Jul 20, 1969 | Sunday | Moon landing (Apollo 11) |
| Jan 1, 2000 | Saturday | Y2K rollover |
| Sep 11, 2001 | Tuesday | 9/11 attacks |
| Jan 1, 2025 | Wednesday | New Year's Day 2025 |
Frequently Asked Questions
How does the calculator determine the day of the week?
The calculator uses JavaScript's Date object, which implements the proleptic Gregorian calendar. It converts the date to a day index (0 = Sunday, 1 = Monday, … 6 = Saturday) using the built-in getDay() method, then maps that to the day name.
What is an ISO week number?
ISO week numbers (ISO 8601) divide the year into 52 or 53 weeks, each starting on Monday. Week 1 is the week containing the year's first Thursday. This standard is widely used in business, logistics, and European calendars. The calculator shows the ISO week number alongside the day name.
Can I look up dates far in the past or future?
Yes. The Gregorian calendar rules are applied retroactively (proleptic Gregorian), so you can look up any date from year 1 to year 9999. Note that before 1582, the Julian calendar was in use in most of Europe, so historical results before that date may differ from primary historical sources.
What day did January 1, 2000 (Y2K) fall on?
January 1, 2000 was a Saturday. The year 2000 was also notable as a leap year — it is divisible by 400, so it had 366 days despite being a century year.
What is the Doomsday algorithm?
The Doomsday algorithm is a mental math shortcut invented by mathematician John Conway for calculating the day of the week for any date. It relies on the fact that certain “doomsday” dates in every year (e.g., 4/4, 6/6, 8/8, 10/10, 12/12) always fall on the same day of the week within that year. This calculator uses the more straightforward built-in Date API for accuracy.