Sagittarius is transparent. You'll never be in doubt about their feelings about anything. They also have all the tact of a sledgehammer, although they are completely well intentioned. They're better with groups than with individuals, and it's fairly certain that you will do something that they will disagree with. They are probably the easiest sign to defuse, and a simple and sincere apology will be enough to start the process.

Sincerely apologize. Ask what can be done to fix things. Chances are good that Sagittarius will respond with their own apology. Your task then is to see what you can do to make sure this particular situation doesn't happen again.

Ask your Sagittarius what it was that really bothered him and encourage him to explore the situation. Unless you've done something really obnoxious, there's a good chance that his upset really wasn't caused by you, but by his own mood.

Share a meal. Either go out to eat at a place you both love, or cook something together. Whatever you do, make it a shared activity, even if it's just deciding on the topping for a delivery pizza. Reinforce that partnership bond.

Relax, watch a movie, have a giggle together, have some fun. Sagittarius finds laughter and shared enjoyment reassuring. What she doesn't like to face is the breaking of a bond, of love or friendship.

Let it go once you've settled things. Although Sagittarius is a warm hearted person who would give a stranger the shirt off their back, he doesn't expect extended periods of deep thought and discussion. Once he thinks something is settled, he'll just be irritated if it's opened back up.