The Cubs haven't made any official announcement about the headline to this post, but that's the only conclusion that can be drawn, following this tweet:
#Yankees at #RedSox on Aug. 18 now a night game for ESPN. #RedSox play at San Francisco the next day.
— Pete Abraham (@PeteAbe) July 23, 2013
ESPN's schedule page for August 18 confirms that Yankees/Red Sox will be their featured Sunday night game that day. (Gee, what a surprise.)
The Cubs/Cardinals game for August 18 is listed as "TBD" on schedules; that same ESPN.com page says the Cubs game will be at 1 p.m. ET (noon CT), but the more likely result will be that Cubs/Cardinals game will be a 1:20 p.m. CT start and be televised on WGN. I'll update this post when that's officially announced by the Cubs.
There's one other "TBD" Sunday game on the Cubs' schedule -- September 1 against the Phillies. Unless the Phillies miraculously find themselves in contention, that one's also likely headed for a 1:20 CT start.