Let us know how talking to him goes! A non-conflict approach to this conversation may be to let him know that you do not think he did this intentionally; in other words, you do not feel like he is out to hurt you on purpose. You are newly married, and there is an adjustment period that we all go through. It takes time to consciously remember that there is another human to consider in decisions. And it takes conscious effort. While dating, we naturally consider each other because we are infatuated and never get our lovers out of our head. To be thoughtless at that time is not common. But put a ring on it, and suddenly you become "one" and that leads to forgetting that the other person is not YOU. lol.
I don't think he did this intentionally. I think he was just a little more oblivious than he can afford to be in the future.
If he has done these trips before, while you were dating, and you had no issue with it then, I would also let him know that it was quite surprising to you to find that it bothered you now. Definitely tell him it was the non-discussion of it prior to making the decision that is bothering you. You want the opportunity to support him, because you love him and want him to be both happy and successful, and he can trust you to negotiate to a win/win solution.
One solution to your B-day that I can think of, besides you hanging out in a men's retreat for five days, is for him to go and settle his clients in, and then take off with you for that night. Are there nice restaurants around there? Is he staying at a nice place? Is it in an urban area, where you can go for an overnight and have him all to yourself for a while? Can someone else from work (perhaps a single counselor, or at least someone who is not a newlywed!) take over the task of escorting these clients for the week? That would be ideal, but just think of things where you each can get enough of what you want to make you both happy.