I honestly think they don't expect most of the kids to be there. They will force the staff to be there, classrooms will be mostly empty, but the state will be satisfied that they had the makeup day.
I would send my kids if I could, but if I had unchangeable plans already in place those would probably take priority.
That wouldn't fly around here. My niece's school district has several Saturdays built into the school calendar this year due to needing to get out of school early to complete a building renovation. However, if 80% or more students do not show up, the day will not count as a school day.
That said, they do not have only 2 days notice for Saturday school. I think that would be really difficult to work around. I would not be very happy with that short of notice. The district should have planned it for further out in the school year so people could plan.