We have been going in the summer holidays ( July or August ) since the park opened in 1992 as EuroDisney. It is still my favourite time, along with early December, which is a great time for Christmas celebrations without crowds and long queues.
Both these seasons have Fantillusion electrical parade after dark every day ( 10.25pm in summer ) followed by Enchanted Fireworks over the Castle to music at 11pm when the park closes.
You will not see these at quieter times, nor during the Easter holidays, which are VERY busy.
With Extra Magic Hours from 8am this gives you 15 hours of ride time, and if you nip in a ride queue quickly before they put the rope across the entrance at 11pm you will still ride, so could end up leaving the park nearer midnight.
Don't waste time queueing for the selected rides during the day that you can do in EMH.
Summer crowds depends on what days and dates you go. Most local people have Francilien Annual Passports, so if you go on the Blockout days when they can't use them, the parks are less busy.
The busiest days in July and August tend to be different from the rest of the year. Saturday and Sunday are still the most crowded, followed by Monday and Tuesday. Thusdays can vary, and Wednesday and Friday are usually the less busy days.
Last August we arrived on a Monday, and had a long queue to get through security checks around 11.30am after checking in. The DL park was extremely busy, so after lunch ( book advance reservations before you go of course, for buffet and table service dining ), we did rides with a large capacity so quicker moving queue, or less popular attractions like the caves in Adventure Isle, Cabine Des Robinson ( Swiss Family Robinson Treehouse ), Mark Twain riverboat which goes around Big Thunder Mountain, Captain Nemo's Submarine Nautilus, and the two rides at the back of Fantasyland which often get missed by people - Storybook boat ride and Casey Junior Train ( both 30 mins queue ).
Small World and Pirates OTC queues may look very long,but they both move quickly as large capacity boats with efficient loading / unloading. POTC lasts about 10 mins, and Small World 8 mins, which is better than queueing for ages for a ride that is over in 2 mins like Dumbo Flying Elephants. ( Dumbo is open during EMH ).
Tuesday was also busy, and on Wednesday it was chilly and wet first thing, so we went to Paris for the day, and had a lovely time in hot sunshine. When we arrived back in DLP around 9.30 pm it had been raining all day, so the park had not been as crowded. Thusday was not too bad, and our last day, Friday was much less busy.
Another poster on the Trip Advisor Marne La Vallee forum went the week before us, ( also Monday to Friday )and they had shorter queues than we did. I think their dates were a Francilien Blockout, and it was also cheaper, as I had priced those dates as well.
You can compare our two Trip Reports on the forum.
Not going this summer unfortunately, as I went in January and am booked again for early October ( Halloween festival ), and going elsewhere in July and November.
I will really miss not seeing Fantillusion and Fireworks this year though.
Have a look at the DLP/ MLV forum on TA. New posters are always welcome.
www.tripadvisor.co.uk/ShowForum-g226865-i2224-Marne_la_Vallee_Seine_et_Marne_Ile_de_France.html