To me, the best time of year to go camping in California is in April and a close second is October. You don't have to fight summer crowds, the weather is typically in a nuetral position and the campsites are not as crowded.
Living in California, the weather is just right, cold enough to enjoy a nice chill at night, and warm enough during the day to hang out in sandles and shorts. If you schedule it in late April, you'll typically avoid any of the late rains. October is a hit or miss as there is typically some rain that may come in the later weeks but for the most part its worth it.
For the second year in a row we decided to head out to Capitola. (New Brighton State Beach in April) We hope that it becomes a yearly occurance as its a really nice experience and place to camp is very close to the beach. There is a lot of shade and the facilities are always in great shape.