Day of the Week Calculator
Enter a year, month and day to find out which weekday it falls on. You also get the day of the year, ISO week number, leap year check and days remaining.
Runs in your browser. Your files are never uploaded.
Loading Day of the Week Calculator…
How to use Day of the Week Calculator
- Enter the year, choose the month and type the day, or press Today.
- Read the weekday in the highlighted box.
- Check the day of the year, ISO week number and days left in that year.
- Change any field to try another date; invalid dates such as 31 April are flagged.
What you get and how it is computed
The weekday comes from counting days since a fixed reference moment in UTC, so time zones and daylight saving changes cannot shift the answer. The day of the year is the number of days since 1 January plus one. The days left equals 365 or 366 minus the day of the year.
A year is a leap year when it is divisible by 4, except century years, which must also be divisible by 400. So 2000 and 2024 are leap years, while 1900 and 2100 are not.
Worked checks: 1 January 2000 is a Saturday and day 1 of a 366-day year. 15 August 1947 is a Friday. 3 January 2021 is a Sunday, and it belongs to ISO week 53 of 2020.
Practical uses
- Checking what weekday a birthday, anniversary or historic event fell on.
- Planning events and deadlines that must avoid weekends.
- Finding the ISO week number used in project plans, manufacturing and logistics.
- Confirming whether a given year has a 29 February.
- Seeing how much of the year remains for goals and budgets.
Limits worth knowing
The tool supports years 100 to 9999 and uses the same Gregorian rules for all of them, which is a mathematical convention rather than a historical record. Week numbering varies by country: the US often starts weeks on Sunday and counts week 1 differently, so the ISO number may not match a local calendar. The calculation is performed entirely in your browser and no dates are sent anywhere.
Why use this day of the week calculator tool
- Any year 100 to 9999. Uses UTC date handling that avoids the JavaScript two-digit year quirk.
- ISO week numbers. Follows the ISO 8601 rule that week 1 contains the first Thursday.
- Leap year detection. Applies the full divisible-by-4, 100 and 400 rule.
- Date validation. Rejects impossible dates instead of silently rolling them forward.
Frequently asked questions
How can I find the weekday of a date by hand?
Use a known anchor. 1 January 2000 was a Saturday, and each normal year shifts the weekday by one while a leap year shifts it by two. Counting days from the anchor and taking the remainder modulo 7 gives the weekday. This tool does that automatically.
What is the ISO week number?
ISO 8601 numbers weeks from Monday, and week 1 is the week containing the first Thursday of the year. As a result, early January dates can belong to week 52 or 53 of the previous year, and late December dates can belong to week 1 of the next. For example, 30 December 2024 is in week 1 of 2025.
Is the calendar accurate for old dates?
It uses the proleptic Gregorian calendar, extending today’s rules backward. Countries switched from the Julian calendar between 1582 and the 20th century, so a historical document may record a different date for the same event.
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