A walkthrough, end to end.
- 1
Pick any date.
- 2
See the day of the week, ISO week number, and day of the year.
- 3
Works for past and future dates within the Gregorian calendar.
Calendar weekday
JavaScript's Date object computes the weekday using the proleptic Gregorian calendar. ISO week number follows ISO 8601.
What you can do with this.
Birthday weekday
Find what day of the week you were born.
Historical dates
What day of the week was July 4, 1776? (Thursday.)
Future planning
Schedule events on a specific weekday.
ISO week reference
Useful for ISO-8601-aware reporting.
Birthday on a Friday?
Plan parties around weekend weekdays.
Astrology/numerology
Some traditions use the birth weekday.
Trivia & history
Quick reference for any historical date.
Day-of-week 2026 — what's current
Standard Gregorian calendar math.
Frequently asked.
Reliable for dates after 1582 when the Gregorian calendar was adopted. Works from JavaScript's allowed date range.
Week 1 is the week containing the first Thursday of the year.
No. All math runs in your browser.
Local zone — check noon if a date sits at midnight UTC boundary.