The prospect of a frost in mid May is not unusual in the Poconos, Lehigh Valley, or the Pinelands in South Jersey -- late season chill is not out of the realm of the norm for a morning or two at this time of the year. It doesn't happen in the middle of May each year but it can happen. Important to note this given we have a shot at a frost early next week once this weekend's upcoming cold front pushes through the region early Sunday.
Best chances for frost appear to be along and north of I-78 in Pennsylvania both Monday and Tuesday mornings -- the GFS indicates the possibility of temperatures getting pretty close to freezing in Allentown and Reading. There's a possibility of patchy frost in South Jersey's Pinelands as well on Tuesday morning as winds will be lightest that morning across that area, allowing for temperatures to drop off into the 30's across interior sections of South Jersey.
For the city and immediate suburbs, frost doesn't look likely. The city's likely to stay above 40 each morning early next week, with the suburbs possibly getting into the upper 30's, especially to the north and west.
Allentown and Millville last had a low at freezing during the month of May in 2010 (32 on May 11th). Philadelphia on that morning was 46 degrees...it's a lot tougher thanks to the urban heat island to get lows below 40 into the city at this time of the year but lows in the lower half of the 40's into Philadelphia for either Monday or Tuesday wouldn't be too tough to pull off.