Names of the days of the week

In many languages around the world, the names of the seven days of the week—Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday—are connected to ancient ideas about stars and gods. This way of naming days began with the Sumerians and was later used by the Babylonians, who passed it to the Roman Empire during late antiquity. These days were often named after the Sun, Moon, and five bright stars that move across the sky.

The idea of a seven-day week was adopted by early Christians from the Hebrew calendar and slowly spread around the world. Sunday became an important day for Christians, and Saturday remained a special day for Jewish traditions, known as the Sabbath. This system became official when the Emperor Constantine made the seven-day week a rule in the year AD 321.

Today, different places start their weeks on different days. For example, the international standard ISO 8601 says Monday is the first day, but in many countries, Sunday is counted as the first day of the week. The history of our weekdays shows how ideas from ancient times continue to shape our lives today.

Days named after planets

Further information: Week and Planetary hours

Between the first and third centuries CE, the Roman Empire changed from an eight-day system to a seven-day week. Early proofs of this change come from writings and markings dating back to the year 60 CE, calling this day "Sunday." The days were named after classical planets in Hellenistic astrology: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn.

The Germanic peoples later changed these names to match their own gods. For example, in Old English, Sunday was "Sunnandæg," meaning "sun's day," and Wednesday was named after the god Woden (Óðinn). These changes happened after the Roman system but before Christianity spread widely.

Numbered days of the week

ISO prescribes Monday as the first day of the week with ISO-8601 for software date formats.

The Slavic, Baltic and Uralic languages (except Finnish and partially Estonian and Võro) adopted numbering but took Monday rather than Sunday as the "first day". This convention is also found in some Austronesian languages whose speakers were converted to Christianity by European missionaries.

In Standard Chinese, the week is referred to as the "Stellar Period" (Chinese: 星期; pinyin: Xīngqī) or "Cycle" (simplified Chinese: 周; traditional Chinese: 週; pinyin: Zhōu). The modern Chinese names for the days of the week are based on a simple numerical sequence. The word for "week" is followed by a number indicating the day: "Monday" is literally the "Stellar Period One"/"Cycle One", that is, the "First day of the Stellar Period/Cycle", etc. The exception is Sunday, where 日 (rì), "day" or "Sun", is used instead of a number.

In the Abrahamic tradition, the first day of the week is Sunday. Seventh-day Sabbaths were sanctified for celebration and rest. After the week was adopted in early Christianity, Sunday remained the first day of the week, but also gradually displaced Saturday as the day of celebration and rest, being considered the Lord's Day.

Mixing of numbering and astronomy

In the Žejane dialect of Istro-Romanian, the names for Monday (lur) and Friday (virer) follow the Latin way of naming days, while Tuesday (utorek), Wednesday (sredu), and Thursday (četrtok) use Slavic naming traditions.

Judaeo-Spanish (Ladino) is mostly based on old Spanish, and the days from Monday to Friday have names very similar to Spanish. Sunday uses an Arabic name meaning "Day one" or "First day" because, as a Jewish language, it avoids using a name linked to a specific religious day. Saturday in Ladino uses the Hebrew name Shabbat, reflecting its importance as the Jewish day of rest.

Other naming systems

The days of the week in Meitei, also called Manipuri, come from ancient stories and beliefs known as the Sanamahi creation myth. In Udmurt culture, the names of the days are tied to old calendar traditions and special rites. For the Yoruba people, the names of the days come from their religion and old beliefs.