PREPARE FOR PLANTS BEFORE TRANSPLANTING OUTDOORS:

If you have started from seeds indoors, you must prepare your plants by “hardening off”, which should be scheduled a week before planting outdoors. Before putting your plant outdoors, be sure that the frost dates have passed if not a cold weather crop. Check labels or use the internet to tell you when you can plant outdoors for certain crops in certain climates. Planting outdoors in mid-May in the Greater Philadelphia area is a safe bet for most plants.
Hardening Off helps the plants to get used to the harsh sun, by giving them more time outdoors each day for about a week until it’s time to plant outdoors.
Click for a video with some different ways to harden off your plants