Pissed

We are invited this wknd to my cousins 50th birthday party and rsvpd over a month ago. I have gone all out to buy a nice dress as I had nothing dressy and it's a cocktail party. I even bought a nice gift yesterday. Now my husband tells me last night he needs to go back home for a month and he plans to leave in a day or 2. I'm upset that he's not buddging to push the date up and leave day after the party. I am tempted to cancel going to the party and return my dress and gift. Whole it's my first cousin I don't know who else would be there that I'd know. I also had in my mind how finally my husband and I have a party to go to and dress up for other than family events. I feel completely bummed. I also don't feel comfortable doing alone. It's a cocktail party I don't drink and I don't know anyone who'd be there