I would say during the week for sure... never on a weekend. A tuesday or Wednesday would probably be the best time to go.
Disneyland is probably the most popular of them all. It has 2 theme parks. Disneyland and California Adventure. Magic mountain is fun but far! Knotts berry farm isn_t all that exciting! Universal studios is probaby the least crowded place of them all! It_s not as popular anymore. Not much to do... good for a half a day. I find City walk (it_s right next to the entrance of universal studios) to be more exciting. Lots of restaurants, shops, and a movie theatre.
All of those places are great to go to if you_ve never been! Of course visit them at least once. Beware all of those places are super expensive. So if you want to save money... go to a near by market and pick up some lunch stuff to pack up for the day. If not, then spend away!
Hope that helped!
Six Flags will probably be relatively uncrowded during Xmas. They are most crowded during summer. I don_t know about Universal, but I expect it will also be less crowded at Xmas than in summer.
Call the parks and talk to guest relations. They might be able to share this info with you.
As far are Six Flags and Universal Studios, the same rule applies. They are less popular but still busy nonetheless!
Have a great time!
Magic Mountain and Universal Studios will be crowded too.
But try going to them Tues-Thurs. It won_t be a bad as going on the mon, fri or the weekend.
Have a great time.
Six Flags and Universal are not as popular as Disneyland. But even here the week between Christmas and New Years will be the busiest, but still more manageable than Disneyland.