If the orders are not clear then you're stuffed.
While Tim is right, the problem is that the public school calendar also states the holidays start on the Monday. So if you go off this NSW Dept of education calendar - the term ends on the Friday - but the holidays start on the Monday.
Is she right - well maybe kinda technically. Is she morally right. Nope.
Friendly advice that isn't of legal nature. So I go back to my original post " the holidays start and finish when mum says so." Best advice I can give you is to be pissed off BUT for christ sake don't let her know you're pissed off. Given she has SPR you're on a hiding to nothing here mate. She is gonna deprive you of 3 days because she can. If you have second half of the holidays next year, I have no doubt she'll tell you the last weekend before school returns is a 'weekend' and not part of the holidays.
I feel for you mate, I really do. My ex used to play exactly the same sorta games. I'm thinking you're a bit early on in this madness. But I slowly learnt that I wasn't gonna win and she loved knowing it. So I stopped trying to have the fight. Trust me. You're not gonna win so when you learn to accept that things get better. Infact after a year or two my ex started offering extra time. I thought SWEET peace at last. BUT NO any additional time given was subtracted at a later date when it suited her...
Sorry my advice isn't any happier.
cheers