Reading of 昨日

I know the word for ‘yesterday’ as kinou きのう The kanji from the dictionary is 昨日 and I’ve seen that elsewhere. But the readings of these kanji don’t seem to make kinou in either On or Kun! 昨 Only seems to have a On reading of サク In fact the dictionary gives one meaning of 作 alone as last year/evening/yesterday, but its not listed as a ‘common word’. And 日 has five readings – but none of them are のう. So how come 昨日 is read きのう?

Kanji characters have a lot of on-readings, and only the most used and let's assume, official, ones are listed in dictionaries. And there are also these "phantom" words which defy the normal ways words are read and these have to be remembered separately as exceptions. You mai use 昨 alone for yesterday (read as saku), but it's very informal and sometimes can be mistaken for something else. It's main reading is saku (i.e. 昨夜 - sakuya = last night), and 昨日 is the only word where both characters have abnormal readings (where normal readings for 日 are: ひ、び、にち、じつ). !!Be careful not to mistake 昨 with 作(=to make/create)!! Another example of abnormal readings would be for example: 流行る - はやる (to be popular, in trend), where normal reading for these characters would be: 流 - なが（す）/リュウ；行 - い（く）、ゆ（く）、コウ.

I would less worry about memorizing on/kun in that manner, it's better if you just try to learn words in their context, on/kun will come to you through experience. Doing it this way will make your fluency better in the long-run.

There are these "pure groups" which have the same reading that are given by a certain compound/primitive which I try to learn them as they are with their readings. However, those which don't belong to such groups are better learned by going with the flow every time you meet them. By constant practicing they tend to stick to you. Well, at least that's how I do it...