The approximate first and last frost dates for Santa Clara County, depending on your location, are November 15 and March 15. These are important dates for gardeners to remember.
First frost date—this is the earliest date you should expect frost to occur. If you have plants that need to be brought in for the winter, or crops you need to pick before frost, this date will be important to you.
Last frost date—after this date, you wouldn't expect any more frosts. It's generally used as a milestone when pruning frost-sensitive plants (such as citrus where you don't want to stimulate delicate new growth until the danger of frost is past).
For our climate, the last frost date is not a reliable metric for when to transplant summer vegetables. Daytime temperatures and soil temperatures are more important. See entries for individual vegetables for specific recommendations.
Also see: Frost—Avoidance and Dealing with Damage