Labor Day is a state holiday.
It's a state civil holiday. I find it curious that the school would be able to find custodial staff willing to come in on a paid holiday (a holiday that is also spelled out in union contracts) but since they'd be paid double time for working on a legal holiday maybe that's the incentive. That makes it even more curious though since budgets are so tight that public schools don't tend to do things that require them to pay their workers double time, especially when teachers are free to go into their classrooms the week before, on regular, non legal holidays where the custodians are already scheduled for normal work hours. Things that make you go hmmm