Men - Black tie
Go through the invitation carefully and check for “black tie”. If it says “black tie” then it means you would need a tuxedo for the party. If you already own a tuxedo, make sure it fits properly and is not out of style and all the accessories are perfectly ok. If you opt for renting a tuxedo then make sure you receive all the other pieces with it like shirt, cuff links, cummerbund, shirt and shoes.
Men - Black tie optional
If the invitation says “Black tie optional” then it means that a formal suit, usually of black or dark blue colour, may be worn in place of a tuxedo. If you choose to wear a suit in place of a tuxedo then make sure the tie gives a formal look.
Men - White tie
If the invitation says “White tie dinner formal” then it means it is a very formal affair and black dress coat / tailcoat along with a formal shirt, vest, white tie, cummerbund, cuff links and shiny black shoes should be worn.
Men - Wedding event
If the formal dinner is a part of a wedding then you should adhere to what the bride or groom has asked you to wear. Also, wearing a white tuxedo with tuxedo pants or formal black pants may also be acceptable, especially if it is a small (personal) event.
Women - Black Dress or evening Gown
Get a black dress for the formal dinner; it could also be knee length or just a little above it. If it is more of a glorified event then you should prefer using a long formal dress. Grab your best accessories, jewellery, hand bag and shoes to match the elegance of the event.