Dr. Stephen, unlike the warmer climates of California, we do in fact have a seasonal slowdown every time it gets cold. As a REALTOR, as well as an investor, I find my best shopping always happens in the winter. There are always fewer buyers, and owners get anxious because activity slows down.
This of course, always means you have better negotiating terms when it is times to purchase. The only negative is that there is usually far more inventory in the spring/summer market.
If you are lookiing for a good bargain, then I would say winter is your best bet. If you want best selection, then March to July would be the best time to search.
If you have any questions about where the best place to find a home in Philadelphia or the Main Line, feel free to let me know.