My question involves labor and employment law for the state of: California

If I have requested the 31st off and it is approved....I am given a schedule showing I have it off....Can my employer change my schedule on the 24th and make me work? are there any laws that protect me from that?

The situation if needed: a co worker said he would work on the 31st in a discussion between himself and the boss. the boss wrote the schedule and posted it. said employee, now scheduled to work, says he cannot make it that day. so on the 22nd the boss is telling 2 of my coworkers that one of them will have to work if neither of them will volunteer. he phrased it "you will have to pull straws". Today the boss informs one of them that they will have to work on the 31st and that no approved time off is guaranteed.

Thank you for any help you can provide...