have a conversation, maybe not right now unless you feel you’re ready, with your guy and tell him how you feel about him not including you, and why you feel that way ie you’re sad/hurt because he didn’t include you and then the why, because it’s an important (to you) milestone and then let him respond and listen to him and then, if you still want to, after he responds tell him that you want to be with him on his birthday because it’s important to you. Now here’s where I’m assuming as I have no idea who either of you are as people but perhaps he sees his birthday like any other day and it is not important to him and he mostly views this as a vacation and a return to cruising overlooking the fact that it’s his birthday as well. In short tell him directly how you feel and why you feel that way and what you’d like to happen going forward, after all of that is said and done you guys should come to an understanding and if possible and he wants it he’ll find a way to include you on the cruise or you’ll come into acceptance with the reason (if there is one) he didn’t include you. Open and truly honest communication is important in every single healthy relationship you’ll ever have in your life. Assuming again but chances are like most of us men, he may be a little not in touch with emotions and empathizing and sees this trip simply just as a vacation and thinks you see it as the same. Chances are he just really did not know that you view his 21st as a very special day and probably thought you didn’t care to come along or not. Hopefully this helps and if anyone has any disagreements with what I wrote please voice them because I truly think this is the best course of action and am I always open to other peoples opinions/ideas/critiques as long as they’re valid ;)