What day of the week a date falls on
What day you were born on, what day your birthday falls on, when a deadline lands. With the week number and the days between it and today.
Your figures
Result
- Day of the year
- 1
- Days left in the year
- 365
- ISO week
- 52
- Year that week belongs to
- 1999
- Days from today
- -9,749
What we assume
- Uses the Gregorian calendar, in force since October 1582.
- The ISO week starts on Monday, and week 1 is the one containing the first Thursday of the year.
- Days from today come out negative for dates in the past.
- Time zones are not considered: the date is read as given.
How it is calculated
The Gregorian calendar repeats exactly every 400 years, and that cycle is what makes any weekday calculable without looking anything up. Those 400 years hold 146,097 days, which is exactly 20,871 weeks: that is why 1 January 2000 and 1 January 2400 fall on the same day.
Day of the year
How many days have passed since 1 January, counting that day. 31 December is day 365, or 366 in a leap year.
The ISO week, which is not the obvious one
ISO 8601 defines week 1 as the one containing the first Thursday of the year, with weeks running Monday to Sunday. It sounds convoluted and it has a practical consequence: the first days of January can belong to week 52 or 53 of the previous year, and the last days of December to week 1 of the next.
That is why we also show "the year that week belongs to", which does not always match the year of the date. It is the most common source of errors when grouping data by week.
Leap years
Years divisible by 4 are leap years, except those divisible by 100, except those divisible by 400. That is why 1900 was not a leap year and 2000 was. The correction exists because the solar year is 365.2422 days rather than 365.25: without the century rule, the calendar would drift a day every 128 years.
Before 1582
For dates before October 1582 this calculator extends the Gregorian calendar backwards, which is what the ISO standard does. In reality the Julian calendar was in force, and the switch was not simultaneous: Spain and Italy adopted it in 1582, Britain in 1752 and Russia in 1918. For old dates, the weekday depends on which calendar you are asking about.
An example
1 January 2000 was a Saturday. It was day 1 of the year, with 365 still to come because 2000 was a leap year, and it belonged to week 52 of 1999 under the ISO standard.
Frequently asked questions
Why can 1 January belong to week 52 of last year?
Because ISO groups whole Monday-to-Sunday weeks and week 1 is the one holding the first Thursday. If the year starts on a Friday, Saturday or Sunday, those first days belong to the last week of the previous year. It is correct, however odd it looks.
Can you work it out by hand?
Yes. Methods like Conway’s Doomsday rule get you there in seconds with practice: you memorise each year’s anchor day and use the fact that 4/4, 6/6, 8/8, 10/10 and 12/12 always fall on it.
Does it work for very distant dates?
For the future, yes: the Gregorian calendar has no expiry and the 400-year cycle repeats indefinitely. For the remote past the answer is correct in the proleptic Gregorian calendar, which is not what people were using at the time.
Do the days from today include today?
No: it is the difference between the two dates. From today to tomorrow is 1 day, and a past date gives a negative number.
Keep calculating
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Updated on 2026-09-11. Calculations run in your browser; nothing you type is sent to a server.