Just don't show up or call in "sick".

Long story, but on a grand jury you only need to serve like a certain amount of the days or something like that, and its not the full amount you're scheduled for. Its totally normal for people to miss days all the time or just stop showing up at some point. Nothing to worry about. Take the day off. No issues. Officially you're supposed to be there, but its actually in conflict with what counts as serving, so yes, you can miss days.

That said.. 20 days is unusual. You won't know your actual schedule until you have which specific grand jury you're selected for, on the day you show up and are selected. Most are 10-14 days.