That match is currently scheduled for June 26 at Kezar Stadium in San Francisco. The date might change -- the Sounders play a league match June 24 in Portland -- but it seems highly unlikely that the Quakes will sell their hosting advantage.
Here's an MLS report on how the two clubs advanced:
Seattle Sounders 5, Cal FC 0 – Cal FC hung tough for a half, but an Osvaldo Alonso penalty in the 50th minute gave the hosts the lead, and the wheels came off for the upstarts soon after. Seattle poured it on in the second 45, finding net five times in total to bring the semipro side's Cinderella run to a crashing end at the Starfire Sports Complex. Seattle, the three-time defending champs, next play on the road against the San Jose Earthquakes.
San Jose Earthquakes 1, Minnesota Stars 0 – The Earthquakes kept their late-game magic going, riding an 85th-minute Steven Lenhart header to a 1-0 win over the pesky Minnesota Stars of the NASL. The loss marks Minnesota's first of the season in any competitions, while it propels the Quakes – who were dominant, but wasteful in front of net – into the quarterfinals. They'll host Seattle at Kezar Stadium in San Francisco on June 26.
And here's an overview from MLSsoccer.com on how its clubs did in Open Cup play tonight.