How do I have to read 明日?
明日
I've seen that the kanji 明日sometimes is read as あした　and sometimes as あす　and I know those two mean the same...
So, could you please tell me What is the difference between those two words? and When do I have to say あした and when あす?

Thank you! :)

Question

How do I have to read 明日?
明日
I've seen that the kanji 明日sometimes is read as あした　and sometimes as あす　and I know those two mean the same... 
So, could you please tell me What is the difference between those two words? and When do I have to say あした and when あす?

Thank you! :)

Answer

in fact 明日 has 3 readings
1  あす
2 あした
3 みょうにち
and reading depends on the combintions in the sentences

Answer

あした and あす are more common than みょうにち.
The level of casualness is あした>あす>みょうにち, but you can use all the three as you like.

Answer

Estoy de acuerdo con Shevelen8. En las conversaciones cotidianas, entre personas cercanas, decimos あした.