The Cubs and Pirates are likely to play in the wild-card game this October. In the meantime, the teams have seven regular-season games remaining and either team could finish in either slot. In anticipation of that, ESPN has chosen the matchup for its final Sunday night regular-season broadcast of 2016, via a Cubs press release:
The Chicago Cubs today announced that the Sunday, September 27 game vs. the Pittsburgh Pirates at Wrigley Field, originally listed as "TBD", has been selected for an exclusive national telecast by ESPN and is scheduled to begin at 7:08 p.m. CT.
There is one more "TBD" game on the Cubs' 2015 schedule, Saturday, September 26 at Wrigley Field, also against the Pirates. According to this Fox-TV baseball schedule, the network will choose two games to be shown at 1 p.m. ET (noon CT) on that date.
It seems likely the Cubs and Pirates would be one of those games, as it is the most attractive matchup of the three "TBD" games on that date. If for some reason the Cubs and Pirates aren't chosen for Fox-TV on 9/26, that game would likely be played at 3:05 p.m. CT.
Stay tuned, as the saying goes.