All three U.S. legacy airlines have San Francisco SFO to San Juan SJU Puerto Rico on sale for $317 (AA) to $319 (DL/UA) round trip for travel in April and May 2016. This fare has wide open availability, seven days per week from April 4 to May 19 with only three Sunday departure dates pricing higher ($375-$395).
American Airlines San Francisco SFO – San Juan SJU Puerto Rico
$316.80 Saturday April 16 – Saturday April 23
This AA itinerary requires an overnight in Charlotte on the return. You can avoid an overnight return by flying back to SFO on a weekday via Philadelphia for $319 round trip on American.
United Airlines San Francisco SFO – San Juan SJU Puerto Rico
$318.30 Friday April 15 – Wednesday April 20
United Airlines $258 base fare for Mileage Plus frequent flyer miles.