Generally, if you call the court in advance and explain the situation, they'll work with you to reschedule.
A few years ago I served on a federal grand jury for a full year, working three days every month. A previously-booked cruise presented a conflict, but our judge excused me for that period (on a federal grand jury a quorum of fewer than the full panel is sufficient to proceed). If the summons is for a petit jury, chances are they'll just postpone the entire service.
But a word of warning: do not under any circumstance ignore a federal jury summons. You will be setting yourself up for a very bad day if you do. On the day our grand jury was empaneled, the judge did a roll call as soon as she took the bench. I was amazed at the number of names that were called with no response. Later in the day after we were put to work, a US Marshal told me that "the boys" had been sent off to the homes and businesses of those who didn't show up to pick them up and bring them before her honor to explain their absences. And this is a judge who enjoys a tremendous reputation for brilliance and fairness--and for tolerating absolutely no nonsense. I'd love to have been a fly on the wall in her courtroom when those people were ushered in.
None of this should be scary, though. Just take care of business ahead of time. Reasonable people will deal with reasonable people reasonably.
And, by the way, I found my year of grand jury service fascinating and rewarding. It is absolutely not true that a grand jury will indict a ham sandwich. We told more than one Assisitant US Attorney to take a hike. On the other hand, we were all terribly impressed with the overall quality of the Federal prosecutors and law enforcement officials we dealth with. We had some high profile cases--a couple you've actually probably heard of--and it's comforting to know that once you get past the public political posturings of the Attorney General and his lapdogs, the day-to-day work by the US Attorneys is intelligent, humane and compassionate, because these are the professionals who stay when the politicians change. They're everything you hope for from the lady with the blindfold.