It looks like the Seattle Sounders won't have to play a weekend game during the fourth round of the U.S. Open Cup, after all. If the LA Galaxy II win their third-round match with PSA Elite, the Sounders will visit the StubHub Center Track and Field Stadium on June 18. If PSA Elite manages to pull the upset, the Sounders would host at Starfire Stadium and the match is currently scheduled for June 11.
While this information seems official enough as it was in a U.S. Soccer press release, the dates are apparently not yet set in stone. When asked about them, Sounders head coach Sigi Schmid suggested their potential match against PSA Elite could still be moved to a later date, most likely either June 17 or 18.
At issue is when the Sounders would want to play their home match, as hosting teams are given the ability to essentially pick the date within a given window. The Sounders' last MLS match before the World Cup break is on June 7. Playing a match earlier in the Open Cup window would allow the Sounders to have a real bye week, as even if they advance they wouldn't have to play another Open Cup match until June 24-25 and their next MLS match wouldn't be until June 28 against D.C. United.
Playing on the later date, though, would keep the Sounders on a more "normal" playing rhythm with at least a week between each game. If the Sounders don't plan on giving their players an extended break either way, this may be the preferred schedule.
In short, you may want to wait until the Sounders' know their opponent before making any plans.