Day of the Week Calculator

Find out what day of the week any date falls on. Also shows day of year, ISO week number, and leap year status.

How the Day of the Week Calculator Works

This tool determines which day of the week any date falls on. It also provides additional information like the day of the year, ISO week number, and whether the year is a leap year.

ISO Week Standard

ISO 8601 defines weeks precisely:

Weeks start on Monday (not Sunday as in the US convention)
Week 1 is the one containing the first Thursday of the year
A year has 52 or 53 ISO weeks

Leap Years

A year is a leap year (366 days instead of 365) if:

It is divisible by 4 -- leap year
BUT if divisible by 100 -- NOT a leap year
BUT if divisible by 400 -- leap year

Examples: 2024 is a leap year (divisible by 4), 1900 was not (divisible by 100), 2000 was (divisible by 400).

The Gregorian Calendar

The Gregorian calendar was introduced by Pope Gregory XIII on October 15, 1582, removing 10 days from the Julian calendar. The calendar was adopted at different times around the world: Britain in 1752, Russia in 1918.

Practical Uses

Planning: check if a date falls on a weekday or weekend
Personal curiosity: discover what day you were born on
Business calculations: determine working days in a period
Historical research: verify the day of past events

Frequently Asked Questions

How is the day of the week calculated for a date?▼

The day of the week is determined using the Gregorian calendar algorithm. JavaScript handles this natively with getDay(), returning 0 (Sunday) through 6 (Saturday). Manual algorithms like Zeller's congruence also exist.

What is the ISO week?▼

The ISO 8601 week starts on Monday, and week 1 is the one containing the first Thursday of January. This means January 1 may fall in week 52 or 53 of the previous year.

When is a year a leap year?▼

A year is a leap year if divisible by 4, except centuries (divisible by 100) that are not divisible by 400. So 2000 was a leap year, 1900 was not, and 2024 is.