My fiance's mom's family is very Catholic and I am not at all. When we went to his uncles wedding in the program they had a note in there about what is "expected" from their guests, catholic or non catholic. It just explained the things that were going on and why the did them and who should and shouldn't take communion (you probably won't have to worry about that if you aren't having a full mass). I have been to other Catholic weddings that were just ceremonies and not mass and they were just like any other wedding I have been to.