Now. You'll thank yourself in the summer when everyone is super stressed out and you're slightly more calm. Slightly.
But really, when you know the content and all that's left is small bits of revision, exam period really isn't that bad. Can never start revising too early.
I make notes, so you can carry those around instead of a giant textbook, also makes sure you don't forget tiny details.
As many have said, check the spec.
Do past papers, all the notes in the world will not help you if you don't do past papers.
If you have any question, make sure you ask.