Rittenhouse Square, 18th and Walnut Sts.,
www.philadelphiaweekly.com [1]
On a series of Wednesdays from 7 to 9 p.m., usually during the month of August, Philadelphia Weekly sponsors concerts in Rittenhouse Square [2]. Everyone comes out with blankets, food, and drinks to sit on the grass and listen to live music. The acts are varied and generally good, but often take second stage to the socializing happening around them.
Links:
[1] http://www.philadelphiaweekly.com
[2] http://www.moon.com/destinations/philadelphia/arts-and-leisure/recreation/parks-and-gardens/rittenhouse-square