Should I be feeling annoyed about this?

Okay…so five days ago my boyfriend and his family took his dad to las vegas to celebrate his dad’s 60th birthday. Tomorrow is his father’s actual birthday…

Now my boyfriend swore up and down that as they’d already celebrated his father’s birthday they weren’t going to do anything on the actual day itself and it was okay to make plans for tomorrow evening for us to do something. it’s been about two weeks since we had a free night together and I was looking forward to it…

So I went ahead and planned a nice evening for us…I brought some movie tickets, made a dinner reservation and made sure all my work was done beforeheand so tomorrow night could be relaxed and calm…

My boyfriend just called me up about twenty minutes ago informing me his mother decided to have another party, told him he had to be there as his cousins were showing up and laid a guilt trip on him about going to said party…

Now…I like to think I am a fairly reasonable human being, but I’m feeling a bit annoyed by this. I’m trying really hard not to be…but I was looking forward to tomorrow evening an awful lot…am I over reacting to this?