Harris County Jury Duty Downtown Information/Question

I have jury duty in downtown on Thursday and this is going to be my first one ever. Anything to expect? I am scheduled to show up at 8 am but would it be a problem if I am 40 min late? as I live far away and it's going to be during rush hour so it's like 1.5 hr dirve for me

Also do they give you any certificate of attendance? Is it possible that it could be a two day process or has it never happened?