Dont under estimate the number of folks who will already be present. The Aires on the mountain and nearby will be jam packed full of folks for at least 1-2 weeks before the tour passes through. The roads will get closed when the police decide to close them, generally, this will be when they feel that there is no more space for more motorhomers/cars wild camping on the mountain. They will just turn you around. This will happen maybe 3 days before the tour passes through. Alpe d' Huez is likely to be extremely popular since one gets to see the tour on the climb twice over.
Whatever the case, the night before the tour, the roads get closed. One can still walk up and cycle up, but its unlikely one will get to the summit, and there will be 10's of thousands of others doing the same.
Personally, I would suggest attempting to find parking in Le Bourg d' Ossians (anywhere, not just Aires), then walking up the road (follow the crowds) to find yourself a spot early doors on the 18th.
Your other option is to do what we shall do, watch from Lac du Chambon - the tour passes over the dam at the western end of the lake after ascending to Alpe d' Huez the first time. You only get to see the tour once, but you'll likely get a far far better view from here than from the Honeypot location that is Alpe d' Huez.
Tour comes over this dam: http://goo.gl/maps/avkxK
Option to watch the descent here: http://goo.gl/maps/j55ua although beware that you are best on a climb to see owt!
Plenty of parking options if the village is chocca (which it will be), i.e: http://goo.gl/maps/LuocB
Oh... and if you get up on the Alpe D' Huez road, beware the time required to get back out again. With a million odd folks and 1000's of motorhomes it might take a day to get back down.
Edited: 12:35 pm, June 21, 2013