I don't know of any "center city" Scottish events specifically. Do you have a copy of the Irish Phonebook (Mid Atlantic Celtic Directory)? The have a listing of MANY highland games (and dates) in the Philly Area.

If you're up for something THIS weekend, we're having the Phoenixville Celtic Street Fair tomorrow from 10 to 6:

http://www.usakilts.com/celticfair.php