I've often emphasized the variability of the weather around here from year to year, and that holds true for the date of the last freeze as well - by last freeze, I mean the last day in winter or early spring when the temperature drops to or below 32^oF. On average in Philadelphia and Wilmington, that date is right about now. But it has varied tremendously over the years.

For example, the last freeze in Wilmington occurred on April 14 last year, but the year before that it was on March 27. In Philadelphia, the last freeze has occurred as early as March 11, but as late as May 11.

Now, a few things to keep in mind about these dates. In more rural areas where it's typically colder at night than these near-city readings, the last freeze usually comes a little later, perhaps by a few weeks. Also, official temperature readings are taken several feet above the surface, but on clear calm nights, it's often several degrees colder right next to the ground than it is a few feet above. So you can get frost on the grass even when official temperature readings come in above freezing.