As times have changed, so have our names for the days of the week. Dating back to the Babylonians, the system was fairly simple: They gave a day of the week to each of the seven celestial bodies they knew of — the sun, moon, and five planets (Mercury, Venus, Mars, Jupiter, and Saturn). Our current naming system comes from an amalgamation of the Babylonian, Anglo-Saxon, and Norse mythologies for those seven main celestial bodies — one of the last remaining vestiges of Norse mythology in our regular vernacular. There’s also a good reason why there are seven days in a week: That’s roughly how long each phase of the moon lasts.
In A.D. 321, Roman emperor Constantine the Great made the seven-day week part of the existing Julian calendar. (This threw the Julian calendar, which had been in effect since 45 B.C., out of whack, but the net effect was positive.) While the days of the week were initially numbered rather than named, this new seven-day system did away with the Kalends, Nones, and Ides distinctions and gave greater clarity to each individual day.
The first day of the week got its name from the first object we see in the night sky: the moon. Formerly spelled monedæi, which comes from the Old English words mōnandæg and mōndæg (literally “moon's day”), it's traditionally considered the second day of the week rather than the first. That links it back to our Nordic friends, who reserved the second day of the week for worshipping Máni, their personification of the moon. The name Mona is also part of a related tradition: It’s the Old English word for “moon,” and girls born on Monday in Ancient Britain were sometimes given this name as a result.
Whether you consider it the second day of the week or the third, Tuesday is named for the god of war. For the Anglo-Saxons it was Tiu, while the Vikings called him Tyr; split the difference and you come up with something close to Tuesday. That also explains why romance languages have similar-sounding names for the day: mardi (French), martes (Spanish), and martedi (Italian) all come from Mars, the Roman god of war.
Another day, another mythological god. Traces of the Latin term dies Mercurii, or “day of Mercury,” can again be found in the Romance languages: mercredi (French), mercoledì (Italian), and miércoles (Spanish). “Wednesday'' itself is derived from the Old English Wōdnesdæg and Middle English Wednesdei, which means “day of Woden” — another form of Odin, the god of all gods in Norse mythology. (Anglo-Saxon paganism owed some of its practices to Nordic culture, hence the crossover.)
If you’re familiar with a certain hammer-wielding god of thunder, you already know for whom Thursday is named: Thor, the popular Norse god. Thursday was called Þūnresdæg in Old English, whereas the Romance languages (like French, which has it as jeudi) deriving from Latin (dies Iovis) name the day after Jupiter. That’s no coincidence, as Jupiter was the Roman god of the sky and thunder, not to mention the king of all gods.
The last day of the traditional workweek derives its English name from a Norse deity, but its origin is a bit murkier than the others. Coming from the Nordic goddess Freyja and the Germanic goddess Frigg, it was called Frīġedæġ in Old English. Confusion sets in when you delve into the theory that the two goddesses are actually one and the same. In any case, both are tremendously important: Frigg was known to be wise and have the power of foresight, while Freyja rode a chariot led by two cats and personified everything from love and beauty to fertility and war. Suffice to say, she’s the most important Nordic goddess.
As for Friday's names in the Romance languages (such as viernes in Spanish and vendredi in French), they derive from the Latin dies Veneris and are named in honor of Venus — who, much like Freyja, is the goddess of love, beauty, and fertility.
This one’s simple: Saturday is named for Saturn. That's because, according to second-century astrologer Vettius Valens, the ringed planet controls the day's first hour. The heavenly body itself is named after the Roman god of wealth and agriculture, and various languages’ names for the day are more similar than most: Sæturnesdæg in Old English, dies Saturni in Latin, samedi in French. A slight exception is German, which has two terms for Saturday: Samstag is the more commonly used, but Sonnabend (“Sun-evening”) is sometimes used in northern and western Germany.
You guessed it: Sunday is named for the sun. In German, Sonntag is Sunday, which derives from sonne, their word for sun. In Latin, dies solis translates as “day of the sun” or “day of Sol,” a Roman sun god. Similarly, Norse mythology personified the sun in the form of Sól, a goddess also known as Sunna (who happens to be the sister of Monday’s Máni, the moon).