Black Tie Question

Can someone please ell me what exactly 'Black Tie' entails and what is 'Black Tie Optional'. Also, can you do a black tie wedding on a friday night? i am sooo confused with all this ettiquette!!!

Black tie usually means that it is required for men to wear a tux and women to wear floor-length gowns..

Black tie optional means that the above dress is encouraged, but not required. However, men should wear dark suits.

I say if you are looking to have a more formal affair, go for black tie optional! Why not??

I was told that 'Black Tie' means tuxedos are expected and 'Black Tie Optional' means that tuxedos or dark suits are accepted.

We are having a BTO wedding on a Sunday afternoon. We wanted a formal wedding and wanted to convey that to our over 100 OOT guests.