What is appropriate wear for Green Bay at the end of August? [View all]

My wife and I went to an Eric Church concert in Green Bay this past September and we both wore flannel shirts over our t-shirts. We fit right in as I'd say several thousand other concert goers also wore flannel that night. The few who weren't wearing flannel stood out.

Not sure but I think flannel would be too warm in August and we want to fit in with the rest of the crowd so looking for suggestions as what to wear.

Edit: We are going to see an Alan Jackson concert at the Resch Center.