Go to Disneyland's Annual Passholders page. Then click the "View Blockout Days" links in the lower left.
Since so many of Disneyland's visitors are annual passholders, you want to go on the days that are blocked out to the most passholders. (FWIW, schools in Southern California tend to get out in mid June and go back in early September.)
The Thursdays and Fridays that are blocked out in early June tend to be good days to go, as a result. The weather is much cooler in June that the rest of the summer, too.
After next week, the SoCal annual passes are blocked out for the summer. Disneyland attendance, honestly, doesn't get that bad on block-out days. It's the non-blockout days that are worst.
Whatever you do, go *before* August 16-17. Those are the days that the blockouts lift on the two SoCal APs, and with a couple weeks to go before school restarts, the parks will be packed then.
Now, Disneyland has a three-day pass for $99 for local residents this summer (blocked out on the Fourth of July), so I anticipate that will increase attendance on those Saturdays that are blocked out to the Deluxe and SoCal APs.
So, any weekday between June 29 and August 14 should be okay. Check out the tour plan pages for Disneyland for more advice.
But if you do the Four Hour tour plan on a weekday during the summer blockout window (next Monday through Aug. 14 this year), you will get as much done as one can reasonably expect during a summer visit to Disneyland.
Walt Disney World's Magic Kingdom
Disney's Animal Kingdom
Disney's Hollywood Studios
Disney California Adventure
Universal's Islands of Adventure
Universal Studios Florida
Universal Studios Hollywood