Go to important casting or friend's birthday?

Ok, so I have a bit of a conundrum. I just got an email from my agency for a really important casting that could potentially be very lucrative for me and I really really need the money right now. BUT, my friend’s 25th birthday is this Wednesday and a few of us took the day off so we could spend it with her and get manicures and go shopping and whatnot. I promised her I would do the Wednesday birthday stuff before I knew about the casting, obviously. However, I could still go to the casting and then meet up with the group later (I would have to miss a good chunk of the day-time activities though).
I’m also orchestrating a huge birthday dinner/celebration for her this weekend, so I don’t think she would be too pissed off if I missed most of the Wednesday stuff, but I still feel bad, since I technically made plans with her first. Any suggestions/advice?

ARGH.