The best time to travel California depends on your travel plans.
If you are planning to explore the Californian coastline you should rather come here in spring or fall because there will be thick coastal fog along the shore from mid of June - end of September.
Most travel guides do not mention the coastal fog you will face in summer in California. It is caused by extreme temperature differences between the shore and the inland.
To avoid a disappointing trip along HWY 1 aka Big Sur postpone your vacations to May or October when it is warm and sunny but not foggy.
California’s Central Valley is pleasant all year long. Most times it is sunny, mild and pleasant. The only exception is the summer with temperatures up to 100-110 degrees.
Going to the Sierra Nevada you have to reckon with heavy snowfall from November - April that most times is causing road closures. The rest of the year the weather is usually nice with moderate temperatures.
Surfers, Snowboarders and Skiers should travel to California from January to March. During that time they are catching the best low tides and swells in surf cities like Santa Cruz and best powder in the Sierra.
Where else if not in NorCal are you able to ski and surf within 2 days. From the shore to the Sierra (or the other way round) it is a 4 hours drive only.
Beach lovers, who are thinking about beach vacations only, shouldn’t plan them in San Francisco! All year round the water is freezing cold and the summers are chilly. There is no way of hitting Ocean Beach without a blanket or fleece - seriously.
Due to its location and the coastal fog, San Francisco’s summer temperatures are lower than in spring, fall or winter pending around the lower 60's.
While visitors are freezing in San Francisco during summer, the locals usually are working on their tan several miles south in Santa Cruz or SoCal with at least 80 degrees Fahrenheit.
If you want to go swimming a wetsuit is the only way to stand the water temperature along the coastline from Mendocino to the Big Sur, except you are hardened. The reason is the cold current, which is finally changing around Santa Barbara. There the water should be warm enough for a swim.
In summary beach lovers should stay between Santa Barbara and San Diego or have to exchange the Pacific with a pool in the inland.
Those who are planning to drive HWY 1 and Big Sur should go from North to South since all vista points are at the ocean side.
This is a common route through California and I recommend starting in San Francisco. Depending the length of your vacations I would end the trip in San Diego or Las Vegas, if you want to see the gambling Mecca or again in San Francisco, including a trip through the Death Valley, Yosemite and/or Lake Tahoe.