It isn't too cold this time of year in South Eastern PA, but can be a bit brisk in the evening. As the wedding date gets closer we will have some weather information here. In the mean time though, I would recommend a nice jacket so that when you leave the reception you have something to put on if it is chilly.
The wedding is semiformal, so anything you would wear to a nice day at church or an evening party would work -- dresses, skirts, or suits for the women, a suit and tie or slacks and blazers with a tie for the men. Tuxes and ball gowns are not required, but tennis shoes and jeans wouldn't be good either :o)