I use memorial day weekend as a guide for planting in central PA. Peppers, tomatoes and cucurbits go in after that date and never before. Has worked out well for me over the past five years.

There are some crops you can plant early, even in weather like we are having (or slightly warmer). If you use a fabric row cover you can plant lettuce, peas, swiss chard, beets, and spinach. Check out sites like Johnny's Seeds and look up row cover options. These covers are easy to put over your plants with a little support (I use supports made of PVC pipe and connectors - pretty cheap and last for a long time). They provide protection from frost and insects and are particularly useful for cold tolerant plants like the ones I listed above. I have grown lettuce right into December using these when we had some of the more mild winters.