To comment on the somewhat random note at the end:
For the days of the week, they're all based on the English names, though you need to do a bit of digging for the source of some of the English names and how they relate. Saturday is among the easier ones like Sunday and Monday, it straight up uses the same kanji as Saturn. The rest are similarly named after planets, but more indirectly as they're named after gods with no other direct common point in Japanese and conveniently complete the 5 classical elements
Tuesday was named after TIu (Tyr) who came to be identified as Mars by the Romans. The kanji used for Mars means fire.
Wednesday is named after Wotan (Odin) who came to be identified as Mercury. The kanji used to represent Mercury means water.
Thursday is Thor's day. Thor is a god of lightning, and in the 5 classical elements, lightning belongs to the wood attribute.
Friday is named for Frigg, a goddess attributed to beauty much like Aphrodite and Venus. The kanji for Venus means gold, which fills in for metal.
Saturday is Saturn, which is represented by the kanji for soil to fill in for the earth element.
I cannot say for sure which way these associations go, or if they happened by sheer chance, but they are all at least loosely based on the English names.
Most interestingly, if you dig a bit more broadly you'll find that the gods the days are named for actually all have some relation to the element used to represent them, much like Thor, that's usually even simpler than the planet names that are easier to find. eg Tyr is a god of justice, and justice used to represented most commonly by flames.