We used to love going in October but each year it has progressively gotten to be a much busier time of year with the addition of Halloween celebrations every weekend and the Gay Days event on the first weekend. I am pretty sure you will find it much less crowded if you can swing going in September. Last year the official Disney's HalloweenTime ran Friday, September 26 through Sunday, November 2, 2008 so decorations had to go up prior to that.
I merely mentioned Haunted Mansion just in case it might be something you would miss. Prior to last year we usually went on it once per trip. However, last year it was one of my DD's absolute favorites and we ended up riding it 10 times! It is housed in an old mansion with ghosts all around as you ride through in a "doom buggy" vehicle. It goes up and down and twists and turns driving you through the entire house. Since it's Disney it's not really scary just all in good fun. If you end up going towards the end of the month you might not miss it. Last year it reopened on the 25th and the year before that on the 20th so who knows!
We went in September in 2007. We saw Fantasmic on a Saturday night.
S. S. Columbia was posting the info for times at the same time I was typing and suggested the Laughing Place link I was going to offer. (Great minds!)

If you especially want to see both Fantasmic and the Fireworks I would suggest going on Friday, Saturday and Sunday so you will have more than one chance to see both shows as there is always a chance that they can get canceled due to high winds. However, if both shows are not at the top of your list I would probably go on Saturday, Sunday and Monday and hope for lighter crowds. Mondays are generally less crowded than Fridays. Better yet is
ter-moo's suggestion of Sunday, Monday, Tuesday if seeing the shows is not your top priority.
The best place to start for an overview of all the rides is on the
Disneyland Resort page.
http://disneyland.disney.go.com/disneyland/en_US/home/home?name=HomePage
Also, our very own Disboard has plenty of info.
http://www.wdwinfo.com/disneyland/disneyland-theme-parks.htm
And another favorite of mine is Visions Fantastic which has a bunch of ride videos you can watch.
http://www.visionsfantastic.com/main.php