Am I wrong for being mad??

So last week my boyfriend and I made plans to go to six flags. I asked him if he wanted to go he said yes. I bought memberships for us and we haven’t been able to go the past 3 weeks because it’s been raining so much. So of course today is suppose to be 90 degrees out and gorgeous so I was actually excited to be going. Saturday’s is our day to go out and do something together since we both work all week and don’t have time to actually do anything. So of course this morning I tell him I’m about to shower and that I’d be on my way and we can go. He then informs me that he wants to go see his dads grave which I have absolutely no issue with so of course we’re going to go so I said okay we’ll go see dad and then go to six flags so then he tells me that six flags isn’t going to happen today because his sister is having a cookout for her husbands birthday and he wants to stay in the area. Like what the fuck. So I got mad and I flipped on him because why make plans with me if you’re gonna cancel last minute! I wouldn’t have cared at all if he told me days ago but he claims he was just told this morning but if you knew it was your brother in laws birthday common sense would be that she’s doing something for him. I know he knew way before hand because his family always talks about doing cook outs days in advance so he should have told me. So then he tells me not to bother coming over until later tonight because I’m not gonna ruin his day with his family. Excuse me?? I drop everything for him, I always put him first before my family. I NEVER see my family do anything with my family on Saturdays because it’s our day. Or if I do I tell him right away that our plans are changing and that’s not the morning of when we’re suppose to do something. Am I being over dramatic for being frustrated with him?