Every year for the past several years there are short periodic sales in the winter and spring months for July travel to Europe. One of these is happening now with Air Canada ticket prices as low as $494 round trip for San Francisco to Barcelona for some departure dates during the month of July. In general, finding deals for June or August departures are far more prevalent, so when a low fare deal comes around for July that is less common. Air Canada has a global sale happening through January 13, 2020.
Air Canada has tickets on sale for around $490 to $511 for departure dates from July 14. If you can fly departing in the first week of August, then Air Canada ticket prices drop to as low as $306 to $333 round trip. These $300+ August fares were easier for me to price through meta-search sites like Momondo.com and Skyscanner.com.
Air Canada website: San Francisco SFO – Toronto YYZ – Barcelona BCN – Montreal YUL – SFO
Skyscanner website: Air Canada San Francisco SFO – Montreal YUL – Barcelona BCN – Montreal YUL – Toronto YYZ (overnight) – SFO