Difference between "in" and "on"

<<Also, the album will be released "on the next month", or "in the next month"?


The album will be released next month.>>

Ah, yes, guess that wasn't the best example.
But I mean like, the album will be released "on May 30, 2009", or "in May 30, 2009"? I'm sure you can skip the "on" and "in", but I'm really interested in knowing which one is correct, if any.

It's similar to "it rained in that day", or "it rained on that day".
I know the "in" is at least more popular, but can someone please say something about this?
Any imput would be appreciated, thanks~

You would use "on" for both cases. I don't know what makes you think "in" is more "popular".

Well, people, books and google make me think that.
Thanks anyway.