Hi Scott,

In modern Japanese 'あす' sounds more official and used mainly in writing, whereas 'あした' is often used in conversation. In classic Japanese 'あした' meant 'morning' and then 'next morning' and then became to mean 'next day'.

What he said :P

"Asu" is often used in songs also, but this is more due to the fact that it has 2 syllables and is easier to adapt to a rythmn as opposed to its specific meaning.