Summoned for jury duty on standby

I received a jury duty notice for standby and was wondering if they let you know the night before if you have to go in or not or if they can tell you last minute the day of you need to go.

I was on standby over a year ago and never got called in but I have a feeling since I wasn't called the first time, they might call me this time :( or maybe not, I don't know because I'm not sure how it works who gets selected and who doesn't.