A question for the lady-types

I have two weddings coming up in September. One is a weeknight wedding, and the other is on a Saturday night. I am singing at the latter.

I need to buy a semiformal dress because I haven't had a decent one in years. The thing about me is that my dresses tend to be all black, all the time. My mother says it's not correct to wear black to a wedding.


...so, what's appropriate? Also, should it be full-length, or will knee-length suffice? I would start e-shopping, but I think this is a try-on type thing.

You know, boys have it so easy.