San Francisco can be visited comfortably at any time of year. But if you had to pick 1 time of year, it would be End of Summer/Beginning of Fall.
August-October are the peak months of the year for beautiful warm weather with the city's Indian Summer in full effect. There are many street festivals and outdoor activities going on during this time of year.
November-January is normally San Francisco's rainy/cooler season with temperatures ranging from mild to chilly (although is rarely drops below 40 degrees Fahrenheit).
February-March is another mild time of year, while April-May brings a nicer Spring feel to the city.
June-July (contrary to what Summers should feel like) is actually the coldest time of year to visit. It is often damp and foggy during the day and cold and windy at night.
If you ask anyone from San Francisco the key to surviving the weather there, they will tell you to pack layers of your clothing. It can often times be hot in the morning and very chilly at night.
