It's Wedding time

Going to a wedding in about one month and I have NO idea what I am going to ware. It is a very traditional catholic wedding, if anyone can give me some tips. See the only real wedding I have been to as been my own wedding and since that wedding was a backyard wedding / BBQ / Pool Party - there was no really dress code. People came the way they felt like, some in dresses and some in shorts and t-shirts.

But this one is FORMAL. I have really no clue what to ware. I have a black dress in the closet, but I am afraid I can't ware it anymore, and also questioning if it is a bit too short?!

And I watched the Sex and the City 2 Movie just a few days ago and I am absolutely obsessed with two of Carrie's outfits:

Are any of them appropriate for a formal wedding???
I especially LOVE the purple dress from the scene at the spice market...
Advice?