Time & Date
Day of Week Calculator
A day of week calculator finds the weekday for a selected calendar date and returns both the weekday name and ISO weekday number.
Quick answer
Enter a date in YYYY-MM-DD format.
What this tells you
- •Enter a date in YYYY-MM-DD format.
- •The result shows weekday name and ISO weekday number (Monday=1 to Sunday=7).
- •Uses proleptic Gregorian calendar logic with date-only interpretation.
How to Use
- 1Select a date.
- 2Click Calculate Day.
- 3Read the weekday result and ISO weekday value.
How It Works
Formula
Parse date -> convert to Gregorian calendar day index -> map index to weekday nameThe tool parses the selected date, computes the calendar weekday index, and maps that index to a weekday label and ISO weekday number.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Find weekday for a birthday
Date2026-05-09
ResultSaturday (ISO weekday 6)
This helps verify planning dates, event schedules, and date-based records.
Common mistakes
- Entering invalid calendar dates
- Expecting timezone-aware date-time conversion instead of date-only weekday lookup
- Assuming this tool uses non-Gregorian calendar systems
Frequently Asked Questions
It uses Gregorian calendar date rules.
ISO weekday is Monday=1 through Sunday=7.
This tool treats input as date-only, so it does not apply local time-of-day timezone shifts.