Most of the grammer schools and secondary schools across the USA, have their summer vacation period from around May 1 until around August 30. That means most US families take their holidays during that period. It tends to be the high tourist season for any family destination in the US. Weather wise, it is not the ideal time for travel to some of the low-altitude desert locations in the Southwest, as they get dreadfully hot during the summer.
Summer is a great time to visit New York. The weather in that northern city is great in the summer. Disneyland in California is not bad during the summer because it is cooled by the Pacific Ocean breezes. It will, however be terribly crowded during the summer school-holidays season.
Winter is OK in California as they have mild winters with cool but not cold weather. It will, however be quite frigid in New York during the winter and snow storms are always a possibility.
I agree with Michelle, Spring or Fall are some of the best times to visit either place. The weather is usually warm in California and typically pleasant in New York with warm days and cool evenings.