Day of the Week Calculator
What day of the week a date falls on, whether it is in the past or the future.
How it works
The weekday is derived from the date itself, not from a lookup table, so it is correct for any date the Gregorian calendar covers — including years far outside the range a spreadsheet will accept.
The page also reports the ISO 8601 week number and the ISO weekday number, where Monday is 1 and Sunday is 7. ISO weeks start on Monday and the first week of a year is the one containing its first Thursday, which is why 1 January can legitimately belong to week 52 of the previous year.
For dates before 1582 the calculation continues the Gregorian rules backwards. That is the proleptic Gregorian calendar, and it is not what people alive at the time would have written — most of Europe was still using the Julian calendar, which by then had drifted ten days.
Examples
What day was I born? Enter any past date and the tool names the weekday, along with the ISO week number and weekday number for that date.
Checking a future deadline. Confirm that a date three months out is not a Saturday before writing it into a contract.
A historical date. Dates before 1582 are answered by projecting Gregorian rules backwards, which is arithmetically consistent but will not match the Julian date recorded at the time.
Practical notes
The weekday is derived arithmetically rather than looked up, so any date the Gregorian calendar covers returns an answer, including years far outside the range a spreadsheet accepts.
ISO numbering runs Monday as 1 through Sunday as 7. Several programming languages number from Sunday as 0 instead, which is a routine source of off-by-one errors when moving data between systems — worth checking which convention a library uses before trusting a stored value.
Dates before 1582 continue Gregorian rules backwards. They are arithmetically consistent but historically wrong: contemporaries used the Julian calendar, which by 1582 had drifted ten days, so a projected weekday will not match a date recorded at the time.
Frequently asked questions
What day was I born on?
Enter your date of birth and the result card names the weekday, together with the week number and day of year.
Why does the same date shift by one weekday each year?
A common year is 52 weeks and one day, so a date advances one weekday each year — and two across a leap day.
Why is 1 January sometimes in week 52 or 53?
Because ISO weeks never split: a week belongs to the year containing its Thursday. If 1 January falls on a Friday, Saturday or Sunday, it sits in the last week of the outgoing year.
Are dates before 1582 reliable?
Arithmetically yes, historically no. They use Gregorian rules projected backwards, which differs from the Julian date recorded at the time — ten days by 1582, and more the further back you go.
Is Monday day 1 or day 0?
Under ISO 8601, Monday is 1 and Sunday is 7. Several programming languages number from Sunday as 0 instead, which is a routine source of off-by-one errors.
How far back can I go?
Any year the Gregorian calendar covers. Before 1582 the rules are projected backwards, which is consistent arithmetic but not the date people used at the time.
Why does the week number sometimes belong to another year?
Because ISO weeks are never split. A week belongs to the year containing its Thursday, so early January can fall in the previous year's week 52 or 53.
Can I check several dates at once?
One at a time here. For a range, the calendar view shows a whole month with weekdays laid out.
What was the day of the week I was born?
Enter your date of birth and the tool names it, along with the ISO week and weekday numbers.
Is there a trick for doing this mentally?
Yes, the doomsday method, which anchors each year to a weekday that several easy dates share. This tool is faster and does not misfire on century years.
What day of the week is most common for a birthday?
Over a long period they even out, but within any given year the weekdays of a fixed date shift forward by one annually, or two across a leap year.