He's going for training from the end of August until the end of September, and is going to miss her birthday by a week or two!

I don't know if we're just going to go ahead and have a small party without him (my mom, neighborhood friends, etc.), or delay her birthday by a couple of weeks so he could be there. We were initially planning on going with him to the training, but I'm only just starting the Johns Hopkins internship, I really don't want to take 5 weeks off a month after I begin, you know? Not very professional.
What would you guys do? Would you just push her birthday back by a few weeks or go ahead with the party anyway?