What is day of the week calculator?
Knowing the weekday of a date helps with planning events, checking historical dates and understanding a calendar. This tool answers instantly for any date.
It also shows the day of the year, the ISO week number used by many businesses, and whether the year is a leap year.
How to use the day of the week calculator
- Choose a date.
- Read the weekday and the other details.
How the day of the week calculator works
The weekday comes from the calendar. The day of the year counts days since 1 January. The ISO week number follows the rule that week 1 contains the first Thursday of the year. A leap year is divisible by 4, except for centuries that are not divisible by 400.
Worked example
Common uses of the day of the week calculator
- Planning events.
- Checking birthdays and anniversaries.
- Reading week numbers in reports.
- Scheduling deliveries.
- Calendar and history homework.
Tips and common mistakes
ISO weeks start on Monday.
Use the age calculator to find the weekday you were born and your exact age.
Background: what to know before you rely on it
Our calendar has seven day weeks, which come from ancient traditions, and months of uneven length that come from Roman history. Because 365 is not divisible by 7, a date moves forward one weekday each year, or two after a leap year, and that is why your birthday is on a different day each year. The ISO 8601 standard numbers weeks so that business reports line up, but note that the first days of January can belong to week 52 or 53 of the previous year.
Private, free and fast
This tool runs entirely in your browser. What you type or choose is not uploaded to a server and is not saved after you close the page, which keeps it fast and private. It is free to use as often as you like. See more in all date and time tools, or browse the full list of tools.
Frequently asked questions
What day was I born?
Enter your birth date and read the weekday.
What is the ISO week number?
A standard way of numbering weeks, where week 1 contains the first Thursday.
What makes a leap year?
Years divisible by 4, except centuries that are not divisible by 400.
Does it work for old dates?
Yes, using the modern calendar.
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Results are for information and education. For legal, financial, medical or security decisions, check with a qualified professional. For legal deadlines, confirm how your contract or court counts days.