i'm just starting kanji , and a bit confused. if onyomi is when two kanji together why is 日曜日 has both of the reading?when do i read the kanji kunyomi even so it's with another kanji?or do i just memorize them as words?
I don't think I have the right answer but in my view most kun+on compound words are kind of common "set expressions" like the days of the week, some time expressions, etc. I think in 日曜日 the last 日 "bi" acts more like a suffix, but I'm not quite certain. My advice is, if you can, try to memorize the kun+on, on+kun or even kun-kun kanji compounds as words, not trying to read and split their meaning and reading because they are very few compared to the onyomi compounds and some are very commonly used. I hope it helps!