June - August
Summer in San Francisco is hands-down the most crowded time in the city—and can also be quite chilly, thanks to the fog and ocean air. (Yes, that famous Mark Twain quote about the coldest winter being a San Francisco summer has much merit.) Make sure to pack at least one sweatshirt or sweater, long pants and a scarf. Also note that San Francisco and surrounding areas have diverse microclimates, so it could be warm (80 is considered a heat wave here) and sunny in one neighborhood and dark and cold elsewhere. Hence, San Franciscans’ predilection for layers. Book your summertime trip as early as possible to ensure you get your first pick of hotels. Ditto for popular restaurants that top your list.
December - February
Winter is decidedly the least-crowded time to visit. (Caveat: Thanksgiving and Christmas/New Year's are an exception to this rule, so plan accordingly.) Make friends with rain and wind, especially in December and January. However, temps don't usually dip below 40, and you're likely to score good deals on hotel rates during winter months. Make sure to pack sweaters and layers, a warm jacket, boots and a hat. Of course, an umbrella is a must. If rain doesn't bother you, visiting during tourist-free wintertime can be nice: You can take advantage of ballet and opera season, plus the Golden Gate Bridge is really cool surrounded by fog.
March - May; September - November
Spring is a lovely time to visit the Bay Area: The rain has moved out and the weather is temperate (50s and 60s) but you still need your layers. No question, though, September and October are the best time to explore because it's San Francisco's "real" summer, with abundant sunshine and warmer temps. Generally speaking, hotel rates may be a little better than peak summer rates, but not significantly. Pack your layers, shades and sunscreen and you're good to go. Again, book hotels and make restaurant reservations as early as possible.