The BEST times to go to Disneyland are:

• September-December, except holiday periods. In September kids are back in school, so the crowds drop off. The weather cools off in late October, though it can be crowded on fall weekends. Beginning in early October, you can enjoy Haunted Mansion Holiday. Beginning in early November, the Christmas decorations and it's a small world Holiday will be up. Decorations stay up until approximately January 3. If you want to visit Disneyland during the Christmas-New Year period, pick whatever day the Rose Parade is held -- it siphons off a lot of tourists and locals, especially in the morning.

• Second week of January-March (except holiday weekends). However, often rides will be closed for refurbishment at this time of year. The first week of January can be busy due to extended holiday vacations.

• Super Bowl Sunday. Everyone stays home to watch the game. Thanks to Tessa L for this hint.

• Columbus Day. It's not widely celebrated in California and few people get this day off.

• Any rainy day. Southern Californians think they'll melt if they get wet. Seriously, the parks are often dead when it rains. A few of the more exposed rides (i.e. Dumbo) may close temporarily if it's really pelting down, but if the rain stops they'll open back up.

• Weekdays, especially if you have to go in the summer or holiday seasons.

• During special events that interest you. Bear in mind that events can attract significant crowds, however.

The WORST times to visit Disneyland are:

• Weekends, year-round. The busiest day at Disneyland is Saturday and the busiest day at Disney's California Adventure is Sunday, so if you have to go on a weekend, reverse those days!

• All summer. Crowds are really bad and it's HOT.

• Thanksgiving weekend. Expect it to be horribly crowded (the week leading up to Thanksgiving is busy, too), though the decorations will be up and the weather is normally very pleasant.

• Weekend before Christmas through December 31. This is a very busy time, so forget about riding every attraction. However, it's a nice time to wander around, enjoy the decorations and the weather, which is usually sunny and cool.

• The week on either side of Easter, and Easter weekend. The parks will be crowded, but the weather will usually be nice.

• Other holidays: Memorial Day weekend, Labor Day weekend, President's Day, Martin Luther King's Birthday, Veteran's Day (November 11) will all be crowded.

• During special events. Some don't create a lot of crowds, but some do.

Some less-known events that may affect crowds, park hours and/or traffic at Disneyland:

• Bats Day - April 30-May 2, 2010. This is an unofficial gathering of Goths, so the park will be filled with offbeat people wearing black. Usually has little effect on crowds. Visit BatsDay.net for info.

• Grad Nites - Usually mid-May through mid-June (2010 dates: May 20, 27 and June 3, 10, 16, 17, 24). A private event for graduating high school seniors is held in Disney's California Adventure, followed by a private party at Disneyland that runs overnight until 7:00 am. Private events are also held at ESPN Zone in Downtown Disney. The parks close to the public on these nights at about 6:00 pm. If you'll be staying at the Grand Californian in one of the rooms overlooking DCA, be aware of the potential for amplified music all night -- you might want to ask for a room on the opposite side from the park.

• Gay Days. First weekend of April and October (2010 dates: April 3-4 and October 1-3). Visit GayDaysAnaheim.com for details. In my experience, this event has almost no effect on crowds.

• Utah Education Association Convention - In October. (2010 dates: October 14-15; 2011 dates: October 20-21.) Because of this teachers' convention each year, most schools in the state of Utah have a 4-day or 5-day weekend. Many Utah residents take that opportunity to travel to Disneyland (some people call it Utah Escapes to Anaheim week), so that long weekend can be surprisingly crowded. Thanks to Heather W for info.

• ABC's Very Merry Christmas Day Parade footage is taped at Disneyland, usually around Veterans Day (2009 dates: November 7-8). Unless you want to watch the production process, you may want to avoid Disneyland that weekend, because it can create crowd problems. The Christmas Day Parade is broadcast on ABC on December 25 each year.

• Cast Member Parties are usually held in early December (2009 dates: December 7-8). The only effect this has on guests is that Disneyland closes to the public early (usually 6:00 pm) on those nights.