The parks don't open to the public until 10:00 am, so you should try to save yourself time (and money) by buying a one-day ticket online before going. Don't bother with the ticket that gives you shuttle service from Paris - the RER (regional train) is inexpensive and it is very easy to get from Paris to DLP by RER - take the A line to Marne La Vallée; the station is literally at the entrance to the parks. Saturday in July is probably going to be pretty bad. I always have the impression they let people (without annual passes or staying on-site) in before 10 which means you could probably get in as far as the castle before the official opening. If it is already crowded when you arrive, on either side of the main-street stores there is a covered walkway that runs the length of main street. Stay on the right-hand one to go quickly to Tomorrowland (Space Mountain, Star Tours and Buzz Lightyear all have fast passes) or the left hand one for other lands (it goes without saying that Fantasyland is straight thru the castle. As far as weather is concerned, be prepared for a variety of weather: hot, chilly, sunny and rain (in general for your trip, not just your day at DLP!).