When you're learning a new language, accumulating vocabulary is a big effort: as Steve Martin said decades ago, "those French have a different word for everything." In Japanese, you're not only learning new grammar and new writing systems, but you're rebuilding a new set of words and meanings from the ground up.
Loanwords are occasional freebies (such as エスカレーター for "escalator") yet may be modified beyond recognition (スマホ, "smartphone") or adapted for local use (ワイシャツ, "dress shirt"). Though the 7-day week is familiar, all the day names are different, so that's 7 more things to memorize...
For days of the week, you get a little break. They're pronounced differently, but the Japanese names are in a way the same as the English names. Almost. But they are cousins. All seven Japanese day names share a common ancestor with their respective English names.
Go back 2,200 years or so to the Han Dynasty, and a trending topic among scholars was the Five Elements (Wu Xing, short for the "five types of chi dominating at different times.") These five are: Fire (火 huǒ), Water (水 shuǐ), Wood (木 mù), Metal (金 jīn), and Earth (土 tǔ). The concept was applied to medicine, politics, astrology, martial arts, and more.
If you add the Sun and Moon to the list of five, you end up with seven symbols:
| 日 | 月 | 火 | 水 | 木 | 金 | 土 |
That might seem familiar. If not, let's add some context:
| 日曜日 | 月曜日 | 火曜日 | 水曜日 | 木曜日 | 金曜日 | 土曜日 |
It still seems a bit coincidental: five plus two is seven, but there's nothing intrinsic about a 7-day period, as there is for the lunar month and solar year. Even the 5-day work week is a relatively modern invention. Did East and West develop this independently, or did one side borrow from the other?
The five Wu Xing elements also map to the five planets that could been seen with the naked eye: Mars, Mercury, Jupiter, Venus, and Saturn. In Greek, the word planet means "wanderer", and the Chinese equivalent 行星 means "moving star". Add the Sun and Moon, and the result was the Seven Luminaries (七曜 qī yào): where 曜 survives in the modern Japanese weekday names.
By the first century BCE, both the Greeks and the Chinese had ascribed astrological meaning to these moving stars. Around 400 CE, the Chinese adopted the Hellenic astrological system for their calendar, resulting in the following associations:
| 日: Sun | 月: Moon | 火: Mars | 水: Mercury | 木: Jupiter | 金: Venus | 土: Saturn |
Around 800 to 1000 CE, the Chinese system was brought over to Japan. (In 1911, China moved to using numbered weekdays: 星期一 for Monday, 星期二 for Tuesday, and so on.)
While the Chinese matched fundamental elements to the planets, the Greeks and Romans did the same with their gods. Mars, the god of war, represented the Red Planet (Mars) and so on:
| 物 | Planet | in Spanish | Non-Roman God | in English |
|---|---|---|---|---|
| 日 | Sun | Domingo | ||
| 月 | Moon | Lunes | ||
| 火 | Mars | Martes | ||
| 水 | Mercury | Miércoles | ||
| 木 | Jupiter | Jueves | ||
| 金 | Venus | Viernes | ||
| 土 | Saturn | Sábado |
Romance languages, like Spanish (above), map day names closely to the Latin planet names (except for Saturday, "the Sabbath", and Sunday, the "Lord's day"). Four of the English names are another step of indirection further, though, as the names of non-Roman gods are used. The Norse god Tyr or (Tiw) maps to Mars and gives us Tuesday. The Germanic god Woden (Mercury) becomes Wednesday (Woden's Day); Thor (Jupiter) maps to "Thor's day"; the English goddess Frigg (for Venus) becomes Friday. Saturday comes from the Roman god Saturn; "Sábado" sounds similar but means "Sabbath". Finally:
| 物 | Planet | in Spanish | Non-Roman God | in English |
|---|---|---|---|---|
| 日 | Sun | Domingo | (none) | Sunday |
| 月 | Moon | Lunes | (none) | "Moon"day |
| 火 | Mars | Martes | Tyr/Tiw | Tuesday |
| 水 | Mercury | Miércoles | Woden | Wednesday |
| 木 | Jupiter | Jueves | Thor | Thursday |
| 金 | Venus | Viernes | Frigg | Friday |
| 土 | Saturn | Sábado | (none) | Saturday |
Yes, 火曜日 for "Tuesday" is not exactly a katakana loanword, but they are connected. "Fire" for the Red Planet makes sense, as does "water" for Mercury. Even though China adopted the weekly calendar from the Greeks, the Five Elements had already existed for centuries. Through these associations, Japanese and English share common origins for each day of the week, as a consequence of astronomers two thousand years ago, on different continents, gazing at the same five wandering stars.
お疲れ様, thanks for reading!