Clare, just wanted to wish you luck with jury duty. It's pretty easy to get it postponed, at least here. Just this year, I was called to appear September 8, but we had plans to be in Maine and not be home until late Sept. 9 (no hotel, no plane tickets; therefore, no proof of vacation). I told them that, and they said my request was approved, and I'd be called later.
I also got it postponed once when I was just starting a new job, and explained I had some training scheduled.
I'm sure if your dad explains, they'll give him a postponement.
I'm the queen of getting called for jury duty (like clockwork every 2/3 years), but I've never had to serve -- they've never needed me. It might also be that even if your dad doesn't get out of it, he might have to call in the night before to see if he's needed, and something will happen (like a plea bargain/settlement or your dad has a high # in the jury pool) and he won't have to come in.
Anyway, good luck with that! We all know that it's not random -- they "know" when it would be wicked inconvenient to serve.
