On a party invitation, how do you tell guests that the event is formal?

Question

On a party invitation, how do you tell guests that the event is formal?

Answer 1

please dress formal

or

it's a formal event (:

Answer 2

say: this is a black tie event

or please wear formal outfits

or just say this is a formal

Answer 3

You would say something like "Formal attire, please" if you want the men to wear a suit and the ladies to wear, say, a tea-length dress, suit, or a dress they might wear to church (in some places, it can be even more formal than that).

If you want the men to wear a tux and the ladies to wear formal dresses (the ones with the little straps, very dressy), you would say that it's "Black Tie".