You won't be expected to know everything on your first day, so don't panic. The chances are you'll spend much of the first week filling in registration forms, attending safety seminars and hanging around waiting to see people (giving you plenty of time for background reading). Your new supervisor is more likely than not a human being rather than a victorian school teacher. If you would like to get a head start in your project, then by all means ask if there are any papers you could be looking at. However, you are likely to be in the lab full time for the next three years, the days of long summer holidays are over, so you might be better off enjoying what remains of your last one.