As far as I understand 明日(あした) is jukujikun (also called semantic ateji), so, as with for example 今日, I would expect for the "reading" to be considered over the whole word, instead of being split into the individual kanji readings as is done with normal words.
But in a lot of places (dictionaries, etc) I see the reading of 明日 split into 明(あ)日(した).
I can see where that might come from, since あ is a normal Kun-reading for 明, but the again した is no valid reading for 日.