Usually end of Jan, so it sounds like you'll miss it. Don't worry - it's not SUPER important, but depending on what course you do, you may want to go see your student services about enrolling into units - until then, when you get your offer, just enrol yourself into random units. You can change your enrollment up until about week 3 or 4 into the semester, so don't worry about locking yourself in! There'll be another session during o-week to help you out with understanding uni.
As for getting your student ID, you can do this ANY time at Monash Connect, in Campus Centre. There'll be information online about where that is (Monash has a map floating around somewhere), but if you're coming from the bus bay, you should see a giant building on your left (if you're coming from anywhere else, this building is the largest building on campus). It's called Menzies. Walk through Menzies, and you'll see another big building (only 2 stories, so not as big as Menzies), and that's Campus Centre.