# Day of the Week Calculator

> Find the day of the week for any date from year 1 to 9999, along with its day of the year, ISO week number and whether the year is a leap year.

Category: Date, time & lifestyle · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/day-of-the-week-calculator

Enter a date to see which day of the week it falls on. Dates use the Gregorian calendar throughout, which is the calendar in civil use today.

Also called: Weekday calculator, What day was I born, Day finder.

## Inputs
- Date (default 1969-07-20)

## Results
- Day of the week
- Full date
- Day of the year
- Days left in the year
- ISO week
- Leap year

## Formula

```
Count the days from a reference date of known weekday (1 January 1970 was a Thursday) and take the remainder after dividing by 7. ISO week = the week (Monday to Sunday) containing that week’s Thursday, numbered from the first week with a Thursday in the year.
```

Where:
- Leap year: divisible by 4, except century years not divisible by 400

## Worked example
1. 20 July 1969 is 165 days before 1 January 1970 (a Thursday).
2. 165 = 23 × 7 + 4, so the date falls 4 weekdays before Thursday: Sunday.
3. Day of the year: 31 + 28 + 31 + 30 + 31 + 30 + 20 = 201.

**Result:** 20 July 1969, the day of the Apollo 11 Moon landing, was a Sunday.

## What the result means

ISO week numbers are used in business planning and in much of Europe; a few days at the start or end of a year can belong to the neighbouring year’s week 1 or week 52/53.

## Assumptions and limits
- The proleptic Gregorian calendar is used for all dates.
- Historical dates before a country adopted the Gregorian calendar (1582 in much of Catholic Europe, 1752 in Britain and its colonies) may have been recorded in the Julian calendar, with a different weekday mapping.

## Questions

### Why can 30 December be in week 1?

ISO 8601 assigns each Monday-to-Sunday week to the year that contains its Thursday. 30 December 2024 was a Monday, and that week’s Thursday was 2 January 2025, so it is 2025-W01.

### Is 2000 a leap year?

Yes. Century years are leap years only if divisible by 400, so 2000 was a leap year but 1900 was not.

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Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/day-of-the-week-calculator