September 16 is the start date of the Halloween Time season this year. All the decorations (which are mainly just in Disneyland and not really in California Adventure) will be up by that date. Haunted Mansion Holiday & Space Mountain Ghost Galaxy will be open. All the Halloween treats and merchandise will be on sale.
Mickey's Halloween Party - a separate ticketed event - begins on September 30, then occurs on Monday, October 3rd, and then on every Friday and Tuesday night until Halloween. The final party is on Halloween night.
Make sure to join us in the Halloween at DLR Superthread, and check out the first 2 posts on Page 1 of that thread for all kinds of information (and links to photos) on the season.