Days of the Week: Names, Origins, and Why the Week Has Seven Days

Last reviewed on May 4, 2026

The seven days of the week — Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday — are older than any country that uses them. The names in English come from a mix of Latin and Anglo-Saxon, with planetary deities standing in for Roman gods who in turn stood in for Greek gods. The seven-day count itself comes from Babylon. This page covers where each name comes from, why a week is seven days long, and why some calendars treat Sunday as the first day and others treat Monday as the first day.

Why seven days?

The seven-day week predates the Romans. It comes from Babylonian astronomy, which identified seven "wandering stars" visible to the naked eye: the Sun, the Moon, and the five planets known to antiquity — Mercury, Venus, Mars, Jupiter, Saturn. Each of those got a day. The Jewish week, framed in Genesis as six days of creation followed by a Sabbath, also runs on a seven-day cycle and spread the practice throughout the Roman Empire. By the time Constantine adopted Sunday as the official Roman day of rest in 321 CE, the seven-day week was already universal across the Mediterranean world.

Other cycle lengths have been tried. The French Revolutionary calendar used a ten-day "décade," and the Soviet Union experimented with five- and six-day weeks in the 1930s. None survived. Seven days happens to fit roughly with a quarter of a lunar cycle (29.5 / 4 ≈ 7.4 days), which is probably why the Babylonians settled on it.

The seven names in English

Sunday — day of the Sun

From Old English sunnandæg, "Sun's day," translating Latin dies Solis. In Romance languages the Christian rename to "Lord's day" stuck (Spanish domingo, French dimanche, Italian domenica). English and German kept the older planetary form. Most US calendars place Sunday at the start of the week — see "Why Sunday or Monday?" below.

Monday — day of the Moon

Old English mōnandæg, "Moon's day," translating Latin dies Lunae. The pattern survives in Spanish lunes, French lundi, Italian lunedì. Monday is the first day of the working week in most of the world and the first day of the ISO 8601 week — see ISO 8601 week numbers.

Tuesday — day of Tiw / Tyr

Old English Tīwesdæg, "Tiw's day." Tiw was the Anglo-Saxon equivalent of Tyr, the Norse god of war and law, who in turn substituted for the Roman war god Mars. Latin dies Martis survives in Spanish martes, French mardi, Italian martedì.

Wednesday — day of Woden

Old English Wōdnesdæg, "Woden's day." Woden (Norse Odin) was the Anglo-Saxon analogue of Mercury, the Roman messenger god. Latin dies Mercurii survives in Spanish miércoles, French mercredi, Italian mercoledì. The English spelling preserves the original "Wodnes-" but the pronunciation lost the first d centuries ago.

Thursday — day of Thor

Old English Þunresdæg, "Thunor's day" — Thunor was the Anglo-Saxon thunder god, equivalent to Norse Thor and Roman Jupiter. Latin dies Iovis survives in Spanish jueves, French jeudi, Italian giovedì.

Friday — day of Frigg

Old English Frīgedæg, "Frigg's day." Frigg was the Anglo-Saxon goddess of love and marriage, the equivalent of Roman Venus. Latin dies Veneris survives in Spanish viernes, French vendredi, Italian venerdì.

Saturday — day of Saturn

Old English Sæturnesdæg, "Saturn's day," translating Latin dies Saturni. Unlike the other weekdays, Saturday kept the Roman planetary name in English instead of switching to a Norse equivalent. Romance languages mostly broke the planetary pattern and named the day after the Jewish Sabbath: Spanish sábado, French samedi, Italian sabato.

The planetary pattern at a glance

Why Sunday or Monday at the start?

Two conventions are in widespread use:

• Sunday-first. Common in the United States, Canada, Japan, and most of Latin America. Calendars on this Site use a Sunday-first grid, which matches what most US users expect to see when they print a calendar.
• Monday-first. Standard in most of Europe, the United Kingdom, Australia, New Zealand, and most of the rest of the world. Also the rule under ISO 8601, which defines the week as starting on Monday and ending on Sunday.

Both conventions agree on the order Sunday → Saturday; they only disagree about which day to draw first. The Sunday-first layout has religious roots — Sunday as the Christian Sabbath, with the working week starting on Monday. The Monday-first layout follows the practical observation that the work week starts on Monday and the weekend is Saturday plus Sunday at the end.

Weekdays vs weekends

In most of the world, the working week is Monday through Friday and the weekend is Saturday plus Sunday. There are exceptions:

• Israel, parts of the Gulf states, and much of the Middle East historically ran a Sunday–Thursday working week with Friday and Saturday as the weekend. Many countries in that region have shifted to a Saturday–Sunday weekend in the last decade.
• In Iran, the weekend is Friday only.
• In Nepal, the weekend is Saturday only.

When counting working days across borders, check the local convention before assuming Saturday and Sunday are both off — see the working days vs calendar days guide.

Why "weekdays" sometimes means just five days

In strict usage, a "weekday" is any day Monday through Friday. In common usage, "weekday" sometimes gets used loosely to mean "any of the seven days." When a contract says "respond within 3 weekdays," that almost certainly means Monday through Friday only — but the safer phrasing is "business days," which also implicitly excludes public holidays.

Common mistakes

• Confusing "weekday" (Monday–Friday) with any day of the week.
• Sorting dates by week number without including the year (see ISO 8601).
• Assuming the week starts on Sunday or Monday everywhere — both conventions are widespread.
• Assuming "Saturday" means "weekend" in every country.
• Treating the seven-day cycle as a Christian invention. It's much older.

Related

• Months of the year — the twelve months and where their names come from.
• ISO 8601 week numbers — the international standard for the week.
• Working days vs calendar days — how to count weekdays and business days.
• 2026 monthly calendars — see the layout in practice.