It's a little complicated. As always. First off, a single "weekday" is a youbi 曜日. Every single weekday ends with that youbi, as if it were a suffix.
- nichiyoubi 日曜日
- getsuyoubi 月曜日
- kayoubi 火曜日
- suiyoubi 水曜日
- mokuyoubi 木曜日
- kin'youbi 金曜日
- doyoubi 土曜日
And yes, the reading is nichi-you-bi 日曜日. I know it may be strange because 日 becomes both nichi and bi in the same word, but it's correct. Those are its on'yomi and kun'yomi readings respectively.
Anyway, in a single "week," shuu 週, we have seven "weekdays," youbi 曜日, and the "weekend," which is the most important part of the week, is called shuumatsu 週末, written with the kanji for "week" and "end."
Now, because these weekdays all end with youbi, people get tired of saying youbi and they start dropping it. Just like in Portuguese, "Monday" is Segunda-Feira, and every day except Saturday and Sunday you get this -Feira suffix. People start dropping it, saying only Segunda instead to say "Monday."
In the case of Japanese, however, it's more complicated than that. As always. Because sometimes they drop the youbi 曜日 entirely, and other times they drop only the bi 日 part. See:
- nichiyoubi. nichiyou. nichi. 日曜日。日曜。日。
- getsuyoubi. getsuyou. getsu. 月曜日。月曜。月。
- kayoubi. kayou. ka. 火曜日。火曜。火。
- suiyoubi. suiyou. sui. 水曜日。水曜。水。
- mokuyoubi. mokuyou. moku. 木曜日。木曜。木。
- kin'youbi. kin'you. kin. 金曜日。金曜。金。
- doyoubi. doyou. do. 土曜日。土曜。土。
However, most of the time you won't see the single kanji ones, the ones without the entire youbi 曜日, unless someone's talking about a lot of different weekdays at once. This is because you wouldn't want to mistake the words for weekdays with some other very common words in Japanese.
Weekdays' First Kanji
If you have been learning Japanese you might have noticed all the names of the weekdays have a first kanji that's also a completely unrelated word when separate. These words are:
- nichi 日 hi 日
- getsu 月 tsuki 月
- hi 火
- mizu 水
- ki 木
- tsuchi 土
Now, I know what you might be thinking: the 7 weekdays were the original power rangers. But, no, that's not it. Not quite. Not exactly. It's not about having "fire" power or "water" power or "moon" power or some other avatar power bullshit, no.
Either way, we can see that the first kanji of each weekday is actually a completely unrelated word, which is why it's almost never abbreviated all the way to the first kanji. One case you might see only the first kanji are in calendars and digital clocks, specially those in Japanese games.
Instead of "Mon" for "monday" you'd see ka 火 for kayoubi 火曜日. It's not a fire alert. It's the weekday.
Why Are The Kanji For Weekdays Those Kanji?
Still, the question remains: why are weekdays in Japanese called like this? How did they came up with this stuff? This weird skill-tree-esque naming. Well, for once, there's actually an understandable reason, almost.
First off, know this: the names of the weekdays in English come from the names of ancient gods. Basically, long, long ago there was given god who was so important people felt the need to name a day of the week after him. And they did that 7 times. So each weekday became "day for God number 1, 2, 3, 4, 5, 6 or 7."
Then, centuries pass, we have no imagination, so instead of coming up with new, cooler names we just translate the old weekday names that were in Latin and Greek to English. Also, as it turns out, those Gods were so important people didn't only name weekdays after them, but also named the goddamn planets after them. So... basically, each weekday in English shares its name with one of the planets in our solar system.
But who cares about English! We are here to learn Japanese! Well, turns out the Japanese people exact same thing, except not exactly, because Japanese and Latin have nothing in common. What they did, instead, was naming their weekdays after the planets, of which five were named after the five elements in the Chinese zodiac (fire, water, wood, metal, earth). See:
- Sunday. nichiyoubi 日曜日
Day of the Sun god. (dies Solis)
Sun: hi 日
- Monday. getusyoubi 月曜日
Day of the Moon god (dies Lunae)
Moon: tsuki 月
- Tuesday. kayoubi 火曜日
Day of the Mars god. (dies Martis)
Mars: kasei 火星.
Fire element: 火
- Wednesday. suiyoubi 水曜日
Day of the Mercury god. (dies Mercurii)
Mercury: suisei 水星
Water element: 水
- Thursday. mokuyoubi 木曜日
Day of the Jupiter god. (dies Iovis)
Jupiter: mokusei 木星
Wood element: 木
- Friday. kin'youbi 金曜日
Day of the Venus god. (dies Veneris)
Venus: kinsei 金星
Metal element: 金
- Saturday. doyoubi 土曜日
Day of the Saturn god. (dies Saturni)
Saturn: dosei 土星
Earth element: 土
And before you say it I know the Moon and the Sun are not planets. That's not the point.
So now you know where the names of the Japanese weekdays come from.