Tell them that they were fully aware of this holiday at the interview and it is their responsibility to find someone to work the shifts. Honestly, show them you have balls otherwise they will treat like dirt until the day you leave.
Best advice here tbh. If you roll over once they will keep doing it over and over and over again.
Managers pull this kind of stuff with rotas ALL the time because they don't plan ahead, realize they're going to be short-staffed, then can't be bothered to do the leg work to fix it.....so they throw the ball in your court instead. If he won't budge then tell him you're going to call HR at head office and ask them for advice on the matter.