Is it ok for my ex to take our sons...?

To HIS other ex's house on a holiday that I can't be present(work), without telling me he's going to? He has a son with his other ex. We have two sons. I had to work that day. It was our, mine and the boys, first holiday that we could not spend together. He and the ex made plans for him, her and the three boys to spend that holiday together at her house. However, he did not mention it to me. At least not until he realized that I had found out and was not happy about it. That was at 10:30 the night before when he finally mentioned anything about the plans for the next day while I would be at work, wishing I could be with my babies. The problem was not that they were doing that, it was his not telling me. We do all try to get along for the kids' sakes. But when he pulls stuff like this, it makes it very hard. They are, and I quote them both, "best friends." He and I just "get along." Much of the reason he and I broke up is because of things like this. Him and her, basically, continuing their "emotional relationship" without actually having to have a real relationship. Treating me like a second class citizen when it came to her(amongst other reasons). Treating our sons like second class citizens when it came to his son with her. Being secretive about, not just her and their doings, but far too many other things as well. But he denies doing any of it, or jusitfies by saying at his age he should be able to do what he wants, they are best friends and he's not going to treat her differently because I don't like it, they do it for their son, I'm too immature, I'm too jealous, I'm too insecrure, etc. I'm not looking for comments on our relationship, our breakup. I'm looking for, Am I wrong to be upset that he made these plans for our sons on a holday that I couldn't be there and NOT TELL ME? It's the not telling me that feels so wrong. Like, if it was completely innocent, why keep it a secret?