# Free online weekday calculator

> Enter any date to find out what day of the week it falls on, its position in the year, its ISO week number, and whether it is a weekend or a leap year day.

Find the day of the week, day of the year, ISO week number, and whether a date is a weekend or falls in a leap year.

**Interactive version:** https://onlinecalculator.me/date-time/weekday/
**Category:** Date & Time

## How to use

1. Enter any date in the input. It defaults to today.
2. Read the day of the week, day of the year, ISO week number, weekend status, and leap year flag.
3. Press "Share with my numbers" to copy a link with your date.

## How each result is calculated

**Day of the week:** The JavaScript `Date` object returns a numeric day index (0 = Sunday through 6 = Saturday). The calculator maps this to the full weekday name.

**Day of the year:** The number of milliseconds from midnight on January 1 of that year to midnight of the target date, divided by 86,400,000 ms, plus one.

**ISO week number:** Follows ISO 8601. The first week of the year is the week containing Thursday. The calculator uses the standard algorithm: shift the day-of-week so Monday = 0, find the nearest Thursday, then compute which week of the year that Thursday falls in.

**Leap year:** A year is a leap year if it is divisible by 4 and either not divisible by 100, or divisible by 400.

## Worked example

Date: July 4, 2025

- Day of the week: **Friday**
- Day of the year: **185** (31 Jan + 28 Feb + 31 Mar + 30 Apr + 31 May + 30 Jun + 4 Jul)
- ISO week number: **27**
- Weekend: **No**
- Leap year: **No** (2025 is not divisible by 4)

## Notes

- The ISO week year can differ from the calendar year for dates in early January or late December.
- The calculator uses UTC-based date parsing to avoid time-zone offsets affecting the displayed day.

## Frequently asked questions

### How is the day of the year calculated?

The calculator counts the number of days elapsed since January 1 of the same year, then adds one. January 1 is day 1; December 31 is day 365 (or 366 in a leap year).

### How is the ISO week number calculated?

ISO 8601 defines the first week of the year as the week containing the first Thursday. Weeks run Monday through Sunday. This means January 1 can be in week 52 or 53 of the previous year, and late December can be in week 1 of the following year.

### What is a leap year?

A year divisible by 4 is a leap year, except century years (divisible by 100) which must also be divisible by 400. So 2000 and 2400 are leap years; 1900 and 2100 are not.

### What counts as a weekend?

Saturday and Sunday. This matches the most common international convention. Some countries treat Friday or another day as a non-working day, but this calculator uses the Saturday–Sunday definition.

### Can I look up historical or future dates?

Yes. The calculator works for any valid Gregorian calendar date the browser's date engine supports, typically year 1 through 9999.

## Sources

- [Determination of the day of the week](https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week) — Wikipedia
- [Zeller's congruence](https://en.wikipedia.org/wiki/Zeller%27s_congruence) — Wikipedia

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