Summer is considered part of the peak-season at
Disneyland Resort (DLR). No one knows what next year will be like, but this year, the crowds have been higher than what has been seen in recent years, due to the 60th Anniversary Celebration. The Celebration will last through Labor Day (September 5, 2016), but things like the new parade (PtN), fireworks (Disneyland Forever) and WoC (Celebrate) are likely to last beyond that date. For the last few years, the "summer" season has started on the Friday before Memorial Day (they haven't announced anything for 2016). The peak period is usually mid-June through mid-August. Towards the end of August, the schedule begins to transition to a non-peak season schedule (i.e., shorter hours, reduced entertainment, etc.). It's also worth noting that the Rivers of America (Mark Twain, Columbia, Canoes, and Tom Sawyers Island), the Railroad, and F! will all be non-operational for much/all of 2016 (due to work for Star Wars Land). The
weather tends to hot, but dry, in Southern California in the summer.
For me, I would think of traveling outside of the summer period because the weather in Florida and Tokyo is pretty miserable in summer (warm/hot, humid, and/or rainy).