The appropriate time to let the person in charge know is when you check in in the morning. Then your name can be moved to the first panels sent to courtrooms to be interviewed. Don’t wait until midafternoon, because if they still have juries to fill they won’t necessarily let you leave early.
Here, in CT state courts don’t call prospective jurors to report on Fridays. If you get a notice to report erev yuntif, you simply go on the judicial website and pick another date within 10 months for jury duty.