The Wrigley concert is listed on Saturday, August 26. The Cubs will be in Philadelphia that day, and the next home game after that is Monday night, August 28 against the Pirates. Again, I hope that’s enough time to get the field in shape.
The Zac Brown Band previously played at Wrigley on September 11, 2015. Their website says there are some presales available from the band:
Presale begins Tuesday, January 17th at 10AM local time. Join the Zamily for acZess to a unique presale code.
General on sale begins Saturday, January 21st at 10AM local time.
I assume there will be a presale for Cubs season-ticket holders, as there usually is for shows like this. Other shows at Wrigley this summer include Tom Petty and the Heartbreakers with Chris Stapleton, James Taylor & His All-Star Band with special guest Bonnie Raitt, Billy Joel, and Green Day.