Day of Week Calculator
Find the day of the week, week number, and day of year for any date.
Did this tool work for you?
How to use this calculator
- 1
Select any date using the date picker.
- 2
The day of the week, ISO week number, and day-of-year appear instantly.
Frequently asked questions
How is the week number calculated?
This calculator uses ISO 8601 week numbering, where week 1 is the week containing the first Thursday of the year. Weeks run Monday to Sunday.
What is "day of year"?
Day of year (also called ordinal date) is the sequential count of days from January 1. January 1 is day 1; December 31 is day 365 (or 366 in a leap year).
Can I look up any historical date?
Yes — the calculator works for any date your browser supports, typically from the year 100 through the far future.
Finding the day of the week for any date
How to use the day of week
This day of week gives you instant, accurate results — no registration or download required. Enter your values above and get your result in seconds. The tool is free, works on all devices, and keeps your data private — nothing is stored or shared.
How the day of week works
The day of week uses standard formulas used in financial planning, budgeting, and investment decisions. Enter your inputs, and the tool calculates the result instantly in your browser. No server-side processing means your data stays on your device. Results update in real time as you change inputs.
ISO week numbering
The ISO 8601 standard defines weeks as starting on Monday and ending on Sunday. Week 1 is the week that contains the year's first Thursday. This means week 1 can start in late December of the previous year, and the last days of December can belong to week 1 of the following year.
Historical day of week
Determining the day of the week for a historical date is surprisingly complex — it requires accounting for the Gregorian calendar reform (1582 in many countries, later in others) and varying leap year rules. This calculator uses the proleptic Gregorian calendar for consistency across all dates.
Day of week: how it works
Date and time calculations underpin project planning, legal deadlines, contract management, and everyday scheduling. This tool handles the arithmetic automatically, accounting for leap years, time zones, and calendar edge cases.
Who uses this tool?
Project managers, solicitors, HR teams, and individuals use it to count days, plan around weekends, and verify contract dates — any situation where getting the timing right matters.
Learn more from an authoritative source:
WikipediaAge Calculator
Calculate your exact age in years, months, and days from your date of birth.
Date Calculator
Add or subtract days, weeks, months, or years to any date to find a past or future date.
Date Difference Calculator
Find the exact number of days, weeks, months, and years between two dates.
Time Calculator
Add or subtract two time values in hours, minutes, and seconds.
Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →