The average date of the earliest 32^oF or lower temperature in the fall is about the first of November around here - earlier than that in outlying areas.

Officially in Philadelphia, the earliest freezing temperature occurred on this date back in 1961, when the mercury dipped to 31^oF. In Wilmington, the temperature's actually fallen to freezing as early as September 27, and below freezing as early as October 1. So we've certainly reached the time of year when it's at least possible to see freezing temperatures, even in city areas.

Of course, the first freezing temperature can also arrive fairly late in the fall. In fact, in two years, 1931 and 1939, October and November were officially frost-free, with the first 32^oF temperature not coming until early December.

Keep in mind that official temperature readings are taken a few feet above the surface. But right at ground level, the temperature is usually lower at night. So you can get frost on your grass or on your car even on nights when the "official" temperature doesn't drop to 32^oF or below.