What is the difference between asu and ashita?


I've seen asu and ashita both used as furigana for the same kanji (明日).

Is there a big, subtle or no difference in meaning and usage?




My dictionary says あす is more polite than あした.

And 明日 has an another reading like みょうにち and this is more polite あす.

I know みょうにち is polite but I didn't know あす is more polite than あした. I think
Japanese who know this difference isn't many.

