Day of the Week Calculator
Find the day of the week for any date, its ISO weekday number and day of the year.
Your inputs
Results
Day of the week
Saturday
- ISO weekday number (Mon = 1)
- 6
- Day of the year
- 185
- Days in that year
- 365
✓Exact calculation — rounded only for display
What this calculator does
What day was your birthday, a historic event or a future deadline?
Formula and method
Weekday from the day number since 1970-01-01 (a Thursday), using the proleptic Gregorian calendar.
Worked example
Calculated by this tool with its default inputs:
Inputs
- Date: 2026-07-04
Results
- Day of the week: Saturday
- ISO weekday number (Mon = 1): 6
- Day of the year: 185
- Days in that year: 365
How to read the results
ISO weekday numbers run Monday = 1 to Sunday = 7.
Assumptions and limitations
- Dates before the Gregorian reform are calculated proleptically.
Questions
What is the day of the year?
The ordinal date: 1 January is day 1.
Methodology and verification
- Result type
- Exact mathematical calculation (rounded only for display)
- Formula version
- 1.0
- Automated verification
- 3 reference and validation test cases, checked against independently computed values. The site cannot be built if any of them fail.
- Editorial review
- Awaiting human editorial review. The explanation was drafted with AI assistance; the formula and examples are verified by the automated tests above.
Privacy
Everything you enter is calculated in this browser and is not sent to Numerith or anyone else. Inputs are added to a share link only if you choose to include them. Privacy policy