When's a cheap day to fly to and return from Las Vegas?
    Sorry. Can't answer that, you need to specify the exact
    day you want to depart and the exact day you want to come
    back, and we might let you look at results + or - 3 days,
    unless you don't specify the exact departure airport.

Google Flights has this: https://www.google.com/flights/?f=0#search;f=SFO,OAK,SJC;t=L...

(Disclaimer: I work for Google, on an unrelated project.)