One of the many appeals of California is that it’s a great destination any time of year. Of course, it still depends what you’re looking for. If you want sun, swimming, and the classic California beach scene then you’ll be disappointed from November through April – even in Southern California.
The weather in Southern California can still be sunny and warm (mid-70s) in the winter months but the water is cold. Think of it more as an opportunity to run in the sand and fly a kite than to go swimming and sun tanning. If hitting the beach is part of your vacation plans then May through October is your best bet.
For skiing, snowboarding, and cross-country skiing the months of December, January, and February are the best.
There are also the regional factors to consider. San Diego can be doused in fog for much of May and June making the beaches a sometimes chilly destination.
San Francisco gets consistent fog from early July until late August. Though nearby Marin, Berkeley, Oakland, and the Sonoma and Napa wine counties will be sunny and hot.
Yosemite National Park and Lake Tahoe are cooler than the rest of the state in summer and colder in winter. Freezing temperatures having been recorded in every month of the year in Yosemite.
Having said all that, if I had to pick one month as the best month to visit it would be September. The crowds have thinned from the summer peak season, the weather is warm, and the fog has cleared from both San Francisco and San Diego.
September also has several great festivals and fairs such as the Los Angeles County Fair, the Monterey Jazz Festival, the Sausalito Art Festival, and (my favorite) the Hardly Strictly Bluegrass Music Festival in San Francisco’s Golden Gate Park.