What Day of the Week Was It?
The day of the week for any date, past or future, and the same date in coming years.
Good to know. The calendar repeats every 400 years exactly, so 2426 will look just like 2026.
How it works
In plain words
Count the days from a date whose weekday is known, divide by 7 and look at the remainder. The calendar follows the Gregorian rules: a leap year every 4 years, except century years not divisible by 400.
The formula
weekday = (days since a known Monday) mod 7 leap year: divisible by 4, and not by 100 unless by 400
Worked example
Apollo 11 landed on the Moon on 20 July 1969, a Sunday. It was day 201 of the year, in ISO week 29.
Sources
No outside data or rates: this is plain arithmetic.
4 automated tests check its answers on every change.
About this calculator
Pick a date and this shows which day of the week it falls on: the day you were born, the day a historic event happened, or what day Christmas, a wedding or a deadline falls on in a future year. The table shows the same date in the next few years, handy for birthdays and anniversaries.
The weekday moves on by one day each year, and by two after 29 February in a leap year, because 365 days is 52 weeks and one day. So a birthday on a Monday this year is usually on a Tuesday next year. The whole pattern repeats every 400 years, the length of the Gregorian leap-year cycle, which happens to be an exact number of weeks.
Dates are counted in the Gregorian calendar, which most of Europe adopted in 1582 and Britain and its colonies in 1752. For dates before a country switched, historians often give the old Julian date, which can be ten or eleven days different, so a weekday for those dates may not match the records.
The week number follows ISO 8601, used across Europe: weeks start on Monday, and week 1 is the week with the first Thursday of the year.
Questions people ask
What day of the week was I born?
Enter your date of birth and the calculator shows the weekday, and which day your birthday falls on in the next few years.
Why does my birthday move one day each year?
A year is 52 weeks and one day, or two days in a leap year, so the weekday moves on by one, or by two after 29 February.
How can I work out the day of the week in my head?
Use John Conway’s Doomsday rule: in any year, 4/4, 6/6, 8/8, 10/10, 12/12 and the last day of February fall on the same weekday. Learn that weekday for the year and count from the nearest one.