I've been up there on 4 occasions. Late june-mid august is the most crowded time, though its not until late june that the entire park is open(mt. cartier opens june 24). Pets are not allowed and camping can only be done at the 3 public sites+the various huts. This applies to the national park area...beyond it theres several reserves which probably have different rules, though most of the mountains are located in the park
Personally I like early june best for two reasons:
-very few people on the trails(two climbs saw noone)
-warm days(but not too hot), yet still heavy snowfields...a nice early summer contrast
Theres also a 4 star hotel and restaurant centrally located near the parks office and campground.
Late summer and early fall is also a nice time because everything is still open, theres usually less rain at this time and less crowds. The park in reality only has a few individual trails that most people hike, most of the park is setup so that one trail goes to several mountains. This becomes a problem in peak tourist times when half of montreal heads up that way.
Its actually kind of funny, the campground had one mens toilet to share between probably a hundred campers. And since the area is culturally french women dont have a problem going and using the mens bathroom. Which
shocked my father quite a bit.....
I should mention theres also group cabins to rent, the gulf of saint lawrence is located only 20 miles by crow flies so you can see it from the mountain tops and visit it if you have a hot day or just want some sight seeing.
I've found that if its clear and sunny there in the summer it can get brutally hot, the actual temperature is probably only around 80 but for some reason theres a very strong sun, in addition the trees/treeline is low so for most of the hikes the suns on you. On the other side of this summer nights in the park seem to almost always be in the 30's and 40's and rainy/cloudy days are almost always cold as humidity is non existent there.
I believe the park has the latest snowfields anywhere east of the mississippi or in the appalacians. In fact one day in june was too cold to hike(summits below freezing/sleeting high wind) and two days later it was in the 80's in the valley.
heres some photos:
http://community.webshots.com/album/191491756UkIJFs
the majority of the pics are from the chic chocs and the coast (a few are PEI 2 are katahdin from I-95)