The winter months of November, January and February are the off season in San Francisco, followed by March and April. The kids are in school and the weather is iffy. November, January and February tend to be cold and rainy, and March is a bit warmer, but still rainy. April is a good off season visit choice: the latter half of April usually has nice weather, highs in the 60's, and it's sunny. But the week of Chinese New Year in February is crowded and busy in San Francisco, and hotel rooms would be scarcer and more expensive. December is high season; lots of people visit then. The week around Easter in April is also busy.