Each Major League Baseball team can appear on ESPN's Sunday Night Baseball five times during a season.
The Cubs have appeared twice already, losing to the Giants in San Francisco May 22, and defeating the Pirates at Wrigley Field June 19.
The team announced Friday that the game against the Seattle Mariners at Wrigley Field Sunday, July 31, which had been listed as "TBD" on original schedules, will be played at in the evening (7 p.m. CT) and be ESPN's featured Sunday night matchup.
The Mariners and Cubs don't meet very often; the last time they played each other was in Seattle in 2013. They haven't met at Wrigley since June 12-14, 2007, nine years ago, so the network probably wanted to take advantage of that unusual occurrence. I'd expect a lot of Mariners fans in town for the weekend.
There are three remaining "TBD" games on the Cubs' schedule, all at Wrigley Field, all against the Cardinals: Sunday, August 14, Saturday, September 24 and Sunday, September 25. It wouldn't surprise me to see at least one of those Sunday games wind up as ESPN's Sunday night telecast.