You should say "bon voyage" specifically at the moment someone is departing for a significant journey, typically one involving travel over a long distance or a vacation. It is a French phrase that literally translates to "good journey" and is used as a sophisticated or heartfelt send-off. The most appropriate time to say it is as the traveler is physically leaving—such as at an airport terminal, a train station platform, or as they pull out of a driveway. It is also common to use the phrase in a "Bon Voyage" card given a few days before departure or at a farewell party. In 2026 etiquette, it is considered more "grand" than a simple "have a nice trip" and is often reserved for special occasions like a cruise, a study abroad program, or an international move. You should not use it when someone is just heading to work or running local errands; instead, save it for departures where there is a sense of adventure or a prolonged absence. If they are returning home, the correct counterpart would be "bon retour."