Probably February
Fall is the best time
Was 4 times there in several years, always full
October
From what I've been hearing recently, the crowds are fairly light right now. People seem to be waiting for Galaxy's Edge (Star Wars land) to open on May 31. Light or heavy, though, your best option is to be there when the park opens. In my (rather exyensive) experience with going to Disneyland, the crowd is usually the lightest right at rope drop. . .though not always. You really can't predict what it's going to be like on any given day. Unless it's raining.
Late autumn
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