We are thinking of taking a family trip in the fall. So I have a some questions
How soon do they decorate for Halloween?
How cold does it get?
How crowded is it? If it is really crowded would the earlier in fall be better?
When do they do the trick or treat parties?
Is this season considered "normal" season will there be water in the river and fantasmic?
Thank you in advance for all of your help!!! 
Check out our Halloween at DL thread, which will give you more info on the dates Halloweentime usually begins and when the TOTP starts, as well as all kinds of other photos and info on Halloweentime at DLR.
http://www.disboards.com/showthread.php?t=2408927
If all goes as it has the last couple of years, the official start of the Halloweentime season - meaning the day that the Halloween fireworks begin, the first day Haunted Mansion Holiday and Space Mountain Ghost Galaxy return and any other seasonal attractions or entertainment such as the Halloween Round-Up in Frontierland commence - should be Friday,
September 24. The Trick or Treat Party generally begins one week later, on the first Friday in October, so it will be
October 1st this year.
It rarely gets cold in September or October. In fact, the last couple of years in October have been HOT HOT HOT for some of the time - even up into the high 90's at some points!! BUT, that said, October is an odd month because it can go either way. While it is usually hot (and very, very dry - hence, all the wildfires around that time of year), it can also suddenly start raining...and then dry up and become 80 degrees!! Or, it can also get very windy. I would not expect to have any rain, and it probably will not be cold at all. Most likely, it will be hot and dry, as usual, but bring jackets and things for the nighttime, just in case. So I would say, in a typical October daytime, it could range anywhere from low 70's to upper 90's - that's how unpredictable it is!
Late September and October are technically considered 'off-peak' season in the
Disneyland books (and as far as hotel rates go), but since they have been making a much bigger deal of Halloweentime in the last few years, crowds are packing in and I don't see how it could be seen as off-peak season anymore. In 2009, it seemed much more crowded in October than it had been the years prior. So, I think DLR tries to make sure everything is up and running as much as possible at that time, to accommodate all the crowds. This year, frankly, I expect there will be even more crowds in DLR, as World of Color will draw in lots of people, as will the revamped Toy Story Midway Mania ride.
As far as I know, Fantasmic should be running and Rivers of America should be filled by Halloweentime. Of course, things are always subject to change.
