Where I grew up it is customary to say "A buen tiempo" which means "you've come at a good time" as in offering food when someone walks in while you're eating. The other person is supposed to reply "buen provecho" which sorta means "Bon Appetit" if they are not interested in what you are offering.

I could never make sense of this habit until one day someone who was eating made an offer I couldn't refuse at the time...only then I knew how valuable this tradition is.