In September, beginning after Labor Day. October begins to get more crowded with the Halloween party and lots of schools have fall breaks. We have been in early to mid September with zero crowds. Down sides: shorter park hours, no Electrical Parade, fireworks only on the weekends (if at all), Fantasmic only on the weekends and ride closures. Very often the Matterhorn is down then, the Haunted Mansion has usually been down getting its holiday redo, and several others. You can go to:
www.riderefurbs.com and see whats projected. But it will be at least one or two headliners. I like low crowds and low prices but miss the longer hours and entertainment. Also during that time we have often gotten the "stay four nights get one night free" at the onsite hotels. In 2005 we got the Paradise Pier for a nightly average of $109 which was a very good deal. Offsite we have gotten rates as low as $40 (that was for the Park Inn now called the Camelot there on Harbor). So yes you can save money!
Other off times are January excluding New Years weekend and MLK weekends. February excluding Presidents Week and Lincolns Birthday weekend. Early March. Late April until Memorial Weekend. November excluding Veterans Day weekend and Thanksgiving week/weekend. All good times crowdswise with the same entertainment schedules and shorter hours I listed above and various ride closures.
And be aware ANY weekend ANY time of year at
Disneyland will have crowds in various levels. Especially if warm, nice weather brings in more locals than normal. So any time you plan if you want low crowds days, you need at least one or two weekdays thrown in.