I'd bet you already know the answer to this, but I'll tell you anyway. It is after they extend the scholarship to you. There will be an oath ceremony that you must attend. They'll ask you if you want to bring family/friend etc. to it as that is the day you will become an officer by direct commission. This is the moment that you will sign the dotted line and make an oath. They will take a picture of you making the oath as proof of your commitment. Good luck.