I'm pretty sure that you're only supposed to go to the law one, right?
There is no longer enrolment in person, as Fyrefly said. Instead, you have a "course information session", following which you can get your ID card.
The former is actually somewhat useful for law. What used to happen (keeping in mind that the introductory unit has changed) was that you'd get course material for it (a whole booklet full of it), as well as a lot of information about the first assessment (worth 30%) from the lovely chief examiner. I'm not sure if something similar will occur this year, but there's a good possibility that it might.
The law faculty takes this session quite seriously, and says that if you can't attend one of the dates listed
here for undergraduate students, you must call 1800 MONASH. Considering that I'm quite sure they don't take attendance, you aren't really obliged to do this, but they do consider it a 'step' in getting into your course.
Your biggest loss out of not attending the first information session (on Friday) is that tickets will be sold for the LSS Camp. If you planned to go to it, depending on how jaffy your cohort is, there is a chance that you'll miss out completely.
The other thing that happens on the day is that you can collect your ID card, after having enrolled online and got all those fancy transaction numbers. See
here under the heading "Collect your ID card". You must do this in person -- and obviously since your photo is being taken, you must go yourself -- but it really can be done at any time before the semester starts afaik.