Philadelphia is gearing up for a monumental Fourth of July celebration, though the nation’s 250th anniversary arrives with a complex emotional backdrop. On Saturday, July 5, a sprawling six-hour free concert titled “One Philly: Unity Concert For America” will take over the Benjamin Franklin Parkway, featuring a star-studded lineup including Christina Aguilera, Will Smith, DJ Jazzy Jeff, the Roots, Jill Scott, Seal, State Property, Kathy Sledge, and Infinity Song