I also find this stuff fascinating in fact I went on a whole research hole on this topic before. Actually the Chinese got their current 7 days through Christianity and so it’s not actually just a coincidence that both Saturday and Saturn and 土曜日 and 土星 have the same origin it’s just one more degree of separation. aka Greek → Roman → Christianity → up to Latin
But, for Japanese it’s Greek → Roman → Christianity → Chinese → Japanese. Anyways here’s a Discord message that I made after doing my research. There was more than just this but I did want to exceed a max length Discord message (no nitro so 2000 characters) the context was a meme showing an anime character’s face on different week days:
Perfect opportunity to launch into how I now know that the days of the week in Japanese are based on the 5 traditional Chinese elements: Fire, Water, Wood, Metal, and earth and then there are also sun and moon which are not elements. The reason the sun and moon are included are because the overall east Asian culture appears to have adopted the seven luminaries (originally a Latin system) style of naming the days of the week which is based on the seven luminaries which are the seven moving astrological objects visible to the human eye (where the sun (日 (ひ (hi (sun)))) is the first star, (they’re also called planets often on a lot of explanations in explaining texts which makes sense since 星 (ほし (hoshi (star))) can also be translated as planet (probably the same in Chinese.)) the moon (月 (つき (tsuki (moon)))) is the second star, Mars (火星 (か・せい (ka・sei (Mars)))) is the third star, Mercury (水星 (すい・せい (sui・sei (Mercury)))) is the fourth star, Jupiter (木星 (もく・せい (moku・sei (Jupiter)))) is the fifth star, Venus (金星 (きん・せい (kin・sei (Venus)))) is the sixth star, and Saturn (土星 (ど・せい (do・sei (Saturn)))) is the seventh star) which ultimately is why they start with Sunday like a lot of the world (which is not how I ordered the list below, I ordered that in the traditional Chinese Element order capped off by the two non elements).
火曜日 (か・よう・び (ka・you・bi (fire week-day)))
水曜日 (すい・よう・び (sui・you・bi (water week-day)))
木曜日 (もく・よう・び (moku・you・bi (wood week-day)))
金曜日 (きん・よう・び (kin・you・bi (gold week-day))) Note: I chose gold as the translation since the Japanese dictionary only shows a translation of gold and not metal even though that is the Chinese element it is based on. (attachment 2)
土曜日 (ど・よう・び (do・you・bi (soil week-day))) Note: I chose soil as the translation as it was the first definition in the dictionary for 土。(attachment 3)
日曜日 (にち・よう・び (nichi・you・bi (sun week-day)))
月曜日 (げつ・よう・び (getsu・you・bi (moon week-day)))
After this message was posted someone did the same thing but in Russian and I thought it was awesome so I’ll also post this here
The Russian side
So, the naming basis of the days for the week in Russian is not as cool as it is in Japanese but here goes. Sunday is considered to be the first of the week and that has to do with religion. After that follows Monday, Tuesday, Wednesday, Thursday, Friday and Saturday comes last.
Воскресенье, (Voskresen'ye) = Sunday. It comes from the word
воскресение, (voskresheniye) which is Russian for ‘resurrection’. It is a reference to the day Christ was resurrected, since according to Christian tradition, that happened on a Sunday.
Понедельник, (Ponedel'nik) = Monday. It’s a compound word, comprised from the preposition
по, (po) and the word
По can mean a number of thing, depending on context, but here it can be translated as ‘in regards to/ on the subject of’ and
неделя simple means ‘week’. Therefore, Monday in Russian simply means ‘on the subject of the week’. Things get a bit boring from here, since most other days simply refer to their position in the week.
Вторник, (Vtornik) = Tuesday. It comes from the adverb that means ‘second’,
второй/вторая/второе, (vtoroy/vtoraya/vtoroye) (there are three versions of it because Russian is a gendered language, the genders being: masculine, feminine and neuter).
Среда, (Sreda) = Wednesday. This one is an exception since it comes from the word
средний/средняя/среднее, (sredniy/srednyaya/sredneye) which means middle, since Wednesday is considered to be the middle of the week
Четверг, (Chetverg) = Thursday. It comes from the adverb that means ‘fourth’,
Пятница, (Pyatnitsa) = Friday. It comes from the adverb that means ‘fifth’,
пятый/п́ятая/пятое, (pyátyy/pyátaya/pyátoye). And lastly,
Суббота, (Subbota) = Saturday. It is named similarly to the Greek ‘Σάββατο’, both referring to the Jewish word ‘Shabat’ which means ‘to rest’ and it refers to the day that God rested after creating the world, according to the Bible.