Hi, Emanuele. My name is Ayuka, I'm a native speaker of Japanese. I'm not a professional Japanese teacher, so if you want an answer only from Japenese teachers, you should skip it. so, this is a very interesting question, I've never thought about how strange it was!! Actually, words like 今日 are called 熟字訓(jukujikun): 熟字(jukuji)=two words 訓(kun)=reading In case of 熟字訓、even though there're two kanji in it, we treat it as one word. so in terms of 今日, '今日' is one set, it can't be divided into 今 and 日 when we read it. When Japanese ancient people tried to find a way to read kanji in Japanese style, they started to pronounce 今日 けふ(kefu). They already started to treat them as one word. But ke-fu was really hard to pronounce, they made it easier to read throughout history. and it was changed into きょう(kyou), so there's no connection between why it is read 'きょう' and the reading of 今(like こん,いま) and 日(like か,にち) actually, there are a lot of 熟字訓 in words regarding days like 今日. So I show you some of examples for your reference. ・昨日　きのう　kinou = yesterday ・明日　あした　ashita = tomorrow ・明後日　あさって　asatte= the day after tomorrow ・明々後日　しあさって　shiasatte= two days after tomorrow sorry for my poor English, I hope it will help you even just a little bit. Thank you for reading this.

I have no idea what the specific answer for this specific word is, but honestly... Japanese is just like that. You see this sort of thing a lot, especially with common words. The more common a word is, the more likely it is to be irregular. Again, while I can't speak with confidence about the origins of 今日 specifically, stuff like that happens because Japanese adopted a pre-existing foreign writing system into their extremely different, pre-existing spoken language. The word 今日 likely pre-dates kanji. When they adopted chinese characters, what were they supposed to do? Get rid of the word they'd used all those years and use a chinese loanword instead? Change the pronunciation just to fit the kanji better? Of course not, that would be ridiculous. It made more sense to come up with kanji that fit the meaning and just keep the original pronunciation, so that's what they did.

Kinda late (more than a year) but I think I got this one. 今日 is what is known as a Jukujikun (熟字訓); a reading of a multi kanji term that can't be separated by each kanji, having an entirely new pronunciation. Hope this helps you or someone else :)

Absolutely not! I did appreciate your effort trying to explain the reasons that may be behind that certain reading. Since it's likely that I will learn many words where the reading is different, for now I won't ask myself too many questions: if I do so, it's likely that my time will be invested in searching the origin of certain words instead of studying the actual language. So yeah, thank you for saving my time!

I hope I didn’t sound unfriendly- it’s a perfectly reasonable question. I struggle with this sort of thing myself. It’s one of the reasons Japanese is such a challenge, but also one of the reasons it’s so interesting. If you’re interested, I’m sure you can find out more about the evolution of the language online.