So before the questions start getting asked about how busy it will be on any particular day of the year ... I figured let's get them all out of the way now.
Whenever the Deluxe pass is blocked-out it will be busy.
The parks are generally in high-capacity mode when Deluxe AP's are blocked out. You can see Deluxe block out days here. http://disneyland.disney.go.com/disn...utCalendarPage
Whenever the SoCal pass is NOT blocked-out on a Sunday.
Expect in influx of SoCal AP's. You can see SoCal block out days here.
Other days to watch out for SoCal passholders is the last Sunday before they are blocked out for the summer. This would be June 22nd. And also the first Sunday they are allowed back in. This would be August 17th.
Weekends between January 7th and May 22nd.
Disneyland is offereing the 2fer deal and the parks will be busy.
The weekend of May 16, 17 and 18th.
This is the last weekend of the 2fer. The parks, especially DCA will be packed.
During Spring Break
Spring Break times run from Mid-March to the end of April. Couple that with the 2fer and you have a busy park.
During any major convention in Anaheim
You can find all convention information here
Here are some of the major ones.
January 17 through 20 - NAMM
January 26 and 27 - Yu Gi Oh Regional
January 26 and 27 - MiceChat 3 year anniversary and Gumball Rally
Weekend of February 2nd - USA Danceport championships
Feb 10 through 13 - CHA Winter Convention and trade show
Feb 22 through 24 - United Spirit Association All Cheer and Dance Competition (avoid at ALL COSTS)
Feb 28 through March 2 - Religious Education Congress
March 7 through 9 - Spirit and Dance Drill National Competition (again, AVOID)
March 12 through 15 - Big West Basketball Tournament
April 3 through 6 - Orange County Boat Show (Don't miss the boat ... show)
April 18 through 20 - American Showcase National Cheer and Dance Competition (do I have to say it again?)
July 6 through 10 - LA Dance Magic Finals
October 10 and 11 - BlizzCon
But as you can see, there is pretty much something ALWAYS going on at the convention center. Which means for people who have to travel, they will bring the family who will spend time at the parks while Mom or Dad is at a convention.
During Major Disneyland events
The weekend before, the week of, and the weekend after July 17th
Disneyland 1/2 Marathon Weekend - Labor Day Weekend
Bats Day Weekend - Generally around the weekend of August 20th. If I had a guess I'd say August 22 through 24 in 2008.
Gay Days weekend - Generally the first weekend in October
Presidents Day Weekend
July 4th weekend
All of July and most of August
Labor Day weekend (but usually the least crowded of the holidays)
Columbus weekend (increasingly busy the last few years)
Veterans Day weekend
The week before and through Thanksgiving weekend
From the week before Christmas and through the first weekend after New Years
Weekends in December
So what to do when it's busy? Get there early. Take it slow. Have fun.