We always found October or early November a good time to go.The weather is still warm but not too hot (it also can be cool enough in the evenings for a sweater or hoodie) and the crowds have really thinned out and we always avoid the park on the weekends no matter what time of year.
Imagine our surprise the time we showed up in early November and the entire park was decorated for Christmas already. We were already in full Christmas mode by the time we got off the plane home! That was such a wonderful and memorable vacation. This time we are going in April to experience some of that warm California sunshine and hopefully enjoy the southern coast a bit more!