you would bake and decorate a fondant cake for a Saturday afternoon wedding?

I am making my cousin's cake, but it's about 300 miles from home. She has offered me her kitchen, but I HATE to decorate away from home (somehow things never work right). I'm thinking I can decorate it Thursday, drive down on Friday, and assemble and finish on Saturday. Is that too long? Will the fondant "seal" the cake so it won't dry out?

Your schedule should work out just fine.
It's a tad longer than I like but it still will work.
Yes the fondant will seal the cake
Keep it as cool as yoiu possibly can.

This should be fine. I have done this several times and the cakes turn out fine