This whole summer's gonna be insane because of the Diamond Celebration (the 60th anniversary) We've gone in August multiple times though, and I'd definitely say August is the best time to go, at least from my experience. Check the calendars on the website to see when the Southern California APs are blocked out and GO ON THOSE DAYS. I can't stress that enough. Last summer we went during August for ten days, and about halfway through our trip, SoCal passes opened up and the crowd levels went up dramatically, it was definitely a marked difference, so that DOES make a difference.
The thing about Disneyland is that it's very much a park for the locals, and so when the local passes are blocked, that's the best time to go. That's another reason why August is the best.
Not to brag or anything, but I am an extremely seasoned Disneyland vet, so if you need any sort of help planning, I'd be happy to give you my advice! Don't know if you've been before or not, but I'd love to help with strategy tips and stuff! (mainly 'cause I'm sooooo sad I probably won't be able to go again this summer...)