The short answer to "What are the best days & times to have an open house?" is anytime you believe buyers will come to see the home.
In San Jose, Cupertino and Saratoga, it seems that between 1-4, 1:30-4:30 and 2-5 on Saturday and Sunday are all good times. I've held afternoon open houses on high-traffic streets such as Bollinger, Johnson and Miller in San Jose with Lynbrook High.
I've also held sunset open house in early evening (5-8) for homes with views and/or beautiful sunsets.