Dear Billy,
Happy birthday in advance!
It’s your 33rd next week, and I do hope your party is a blast. I'm so sorry, but I won't be able to make it to your party next week. I just found out this morning that I have to go on an urgent business trip. My boss and I are flying out early next week to meet with some new suppliers. We were supposed to meet with them a while ago, but the meeting was postponed. They suddenly contacted us again late last week and said this was the only time they were available. Since this is important for the company, the trip is unavoidable.
I'm really upset that I'll miss your party. I was really looking forward to catching up with you and everyone else. It feels like ages since we've all been together. Let's definitely make plans for when I get back. How about dinner the following week? We can celebrate your birthday properly then, even if it's a little belated.
Have a wonderful birthday, and I'll see you soon!
Your buddy,
Roddy