If they know there's a long case coming up, those selected for jury service at that time are sent a questionnaire to complete. It asks about your availability, holidays and medical appointments; which you need to evidence. They will then take that into account, but unless you are ineligible for...
I'm currently doing jury service. First day was Monday, sat around in a room for most of the day and wasn't picked for a long 8 week trial. That afternoon, and subsequent afternoons, I've received an email to say that I don't need to report to the court the following day. So have had 3 days off...