Dreaming of Christmas in the City by the Bay? Or a Christmas in sunny San Diego? United and Virgin/Alaska have dropped prices on nonstop flights from San Diego to San Francisco (and vice versa!) from mid-Nov through June - including Christmas and NYE! - for a mere $117 RT. Delta's joined the party with select dates at $134 RT (w/ a stop).
Note that the United fares are basic economy, so no seat selection, credit toward elite status, or carry on bag - you'll have to drop the $ to check a bag. To upgrade to regular economy will be an additional $80 pp for $196 RT. This not the case for Virgin/Alaska.
Booking: search on Google Flights for dates then book direct.
Travel: Mid-November through June including Christmas and NYE (Thanksgiving excluded)
Airlines: United (basic economy), Virgin/Alaska, Delta ($134 RT)