Day of the Week Calculator

Pick any date and the calculator returns its weekday, the day number within the year, the ISO week number and the days remaining until December 31. It works for dates far in the past or future using the Gregorian calendar, which repeats exactly every 400 years. For example, July 4, 1776 was a Thursday, and December 25, 2026 falls on a Friday.

ISO 8601

ISO 8601 is the international standard for writing dates and times. It orders date parts from largest to smallest, as YYYY-MM-DD, so 2026-11-03 can only mean November 3, 2026, avoiding the confusion between American month-first and European day-first formats. Under ISO 8601 the week starts on Monday, and week 1 of a year is the week that contains the year's first Thursday, so a year has 52 or 53 weeks and a few days around New Year can belong to the neighboring year's week numbering. Days are numbered 1 (Monday) through 7 (Sunday), giving week dates such as 2026-W45-2.

Gregorian Calendar

The Gregorian calendar, introduced by Pope Gregory XIII in 1582, is the civil calendar in nearly every country today. It corrected the Julian calendar's slight overestimate of the year by skipping three leap days every 400 years: century years are leap years only when divisible by 400, so 2000 was a leap year but 1900 was not. Because a 400-year cycle contains exactly 20,871 weeks, weekdays repeat on the same dates every four centuries.

Where do Their Names Come From?

The seven-day week traces back to ancient Mesopotamia, and the Romans named the days after the seven classical planets, which were in turn named for gods: the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn. Romance languages kept many of these names, which is why Tuesday is mardi (Mars) in French and Wednesday is miércoles (Mercury) in Spanish.

English took a different route. Germanic peoples replaced most Roman gods with their own counterparts, and the Old English names survived. Sunday and Monday still honor the Sun and the Moon. Tuesday belongs to Tiw, a Germanic god of war matched with Mars. Wednesday is Woden's day, the chief god also known as Odin, equated with Mercury. Thursday is Thor's day, the thunder god paired with Jupiter, and Friday is named for Frigg, a goddess linked with Venus. Only Saturday kept its Roman god, Saturn. Many other languages simply number the days: in Portuguese, Monday is segunda-feira, the second day, and in Chinese the weekdays are numbered one through six.

Trivia About Each Day

Each weekday has picked up its own associations. The table collects a few facts for each one, from the origin of its name to customs and records tied to it.

Whether the week starts on Sunday or Monday depends on where you are. Calendars in the United States, Canada and Japan usually begin with Sunday, following Jewish and Christian tradition in which Sunday is the first day, while ISO 8601 and most of Europe start the week on Monday. In much of the Middle East, the weekend falls on Friday and Saturday instead of Saturday and Sunday.

Because a common year is 52 weeks plus one day, a date moves forward one weekday each year, and two after a leap day. A calendar can therefore be reused when the year type and starting weekday match: 2026's calendar is identical to 2015's and will be used again in 2037.

Frequently asked questions