Day of the Week Calculator
Find which day of the week any date falls on, past or future.
About the Day of the Week Calculator
Enter any date to see its weekday, its ISO week number and its day number in the year. Use it for birthdays, historical events, or planning when a future date falls.
The calculator uses the proleptic Gregorian calendar (the modern calendar extended backwards). Historical dates before a country adopted the Gregorian calendar (1582 in much of Europe, 1752 in Britain and its colonies) may have been recorded in the Julian calendar.
The formula
Weekday from days since a known reference date, modulo 7
Worked example
The first Moon landing, 20 July 1969
- Sunday
- ISO week 29 of 1969
- Day 201 of the year
Questions people ask
What day was I born?
Enter your birth date.
Why might an old document disagree?
It may use the Julian calendar, which by the 1700s ran 11 days behind.
What is an ISO week?
Weeks start on Monday and week 1 is the week containing the year's first Thursday, the standard used in business and in much of Europe.