Together with alanscottevil and bz_mahrabu, I just figured it out. Every year on or around October 29th and February 13th, the sun will rise over Center City Philadelphia exactly aligned with the street grid. And every year on or around April 22nd and August 20th, the sun will set over Center City Philadelphia exactly aligned with the street grid. (This assumes that Philadelphia is rotated 17 degrees clockwise from true cardinal directions, which I think might be true, but I've lost the sketchy-looking page that I Googled that said that.)
On August 20th, I say we party. Who's with me?