Depends what you want out of your holiday.
Drinking/Partying (might as well stay home)
Camping/Surfing (bring a car that you don't mind getting dirty)
Tourist stuff (Once again what do you like to do?)
Also as above are you driving over or flying? Without transport Adelaide is well :cry:
I would say come in March because it gets busy with plenty of stuff to do with the girl like the Fringe Festival, Womad, Festival of arts, Clipsal 500. If you drink wine take a day trip to one of the many regions. Them are just a couple of ideas.