When is it appropriate to button a sport coat?

If its a 3 button jacket, if you must button…the top 2. Never button the bottom. You can get away with the middle only, if you are wearing it real casual with jeans. But you can’t go wrong with the top 2. Maybe start it buttoned then open it up. The jacket should fit right so if lays right when opened. If you are heavier set and your jacket isnt tailored right it will tend to flap open justifying the buttoning. I button and unbutton through out the night when I wear a sport coat.

Is it for a specific event?

This is not for a specific event, I was just curious because I just returned from a visit to France—where it is more common to dress up a little bit—to spend some time with my sister. While there, I wore my sport coat far more often than I do here in the states, always opened.

when you enter the event i think is most appropriate time to have your coat buttoned. all other times there is no need. when getting a blow job at some of these events i feel having my coat buttoned makes me feel much more important for some reason.

you’re asking a question of etiquette, and with sport coats it follows the rules for a suit. essentially, you’re asking how to wear a jacket, as part of a suit or not. buttoned when standing.

you can be less formal, and that would be unbuttoned. always buttoned when entering a room / party / restaurant the first time, when meeting someone, et cetera. unbuttoned when being more active