Its done differently here in Canada--when you get the notice in the mail you just call and tell them "sorry I was planning on going to the beach that day" and they apologize for bothering you that's the end of that.
well not really--I've been summoned twice and boy I seen a lot of folks say anything to get out of it but both times all the cases were settled out of court before juries were needed. A friend of mine got on a murder case and missed about 6 weeks of work(not in a row). I know its your duty as a citizen but I was glad to get off scott free both times