Day of the Week Calculator
Enter any date to find out what day of the week it falls on. Works for any date — past or future — including historical dates and far-future dates.
What Day Was I Born?
Enter your date of birth above to find out what day of the week you were born on. You'll also see the week number of the year, what day of the year it was, and a mini calendar of that month.
Notable Day Lookups
Some popular dates people check:
| Date | Day |
|---|---|
| January 1, 2000 | Saturday |
| July 4, 1776 | Thursday |
| December 25, 2026 | Friday |
| January 1, 2030 | Tuesday |
| February 29, 2028 | Tuesday |
How Day of the Week Is Calculated
The Gregorian calendar repeats its day pattern on a 400-year cycle (146,097 days = exactly 20,871 weeks). This means any date — however far in the past or future — has a mathematically deterministic day of the week.
The classic algorithm for this is Tomohiko Sakamoto's formula (1993), or equivalently Zeller's congruence. For dates from October 15, 1582 onwards (when the Gregorian calendar was adopted), the calculation is exact. For earlier dates, the result reflects the proleptic Gregorian calendar — the same rules projected backwards.
Frequently Asked Questions
What day of the week was Christmas in [year]?
Enter December 25 and any year into the calculator to find out instantly.
Does this work for dates before 1900?
Yes. The calculator uses the proleptic Gregorian calendar for all dates. This matches the calendar system in common use today, projected backwards. Dates before the Gregorian reform of October 1582 would have used the Julian calendar historically, which can differ by a few days from the Gregorian result.
What is the ISO week number?
The ISO 8601 week number starts on Monday, and week 1 is the week containing the year's first Thursday. This means January 1 can be in week 52 or 53 of the previous year if it falls on a Friday, Saturday, or Sunday.
