Day of the Week Calculator
Find which day of the week any calendar date falls on, plus its ISO weekday number and day-of-year count.
- Day of the week
- Saturday
- Weekend?
- Yes
- ISO weekday number (1=Mon ... 7=Sun)
- 6
- Day of the year
- 213
How it works
The date is normalized to midnight UTC first (rebuilding it from its own year, month, and day rather than trusting the browser's local timezone), so the calendar day being measured never shifts depending on where the calculation runs. From there, the weekday simply falls out of the proleptic Gregorian calendar's day-counting rules.
Alongside the weekday name, this also reports the ISO 8601 weekday number (1 for Monday through 7 for Sunday — deliberately different from JavaScript's own native convention of 0 for Sunday, to match the international standard) and the day-of-year count (1 through 365, or 366 in a leap year).
FAQ
How is this different from the ISO Week Number calculator?
That calculator finds which numbered week (1-53) a date falls in for a given year. This one finds which single weekday (Monday through Sunday) a date falls on, plus where it sits in the year as a whole — a different, complementary question about the same calendar date.
Why compute this in UTC instead of my local time?
A calendar date like 'August 1, 2026' has no time zone of its own — it's just a day on the calendar. Normalizing to UTC internally avoids a subtle bug where parsing the same date string could land on the day before or after depending on the browser's local timezone offset.