White Christmas in Philadelphia

Philadelphia's biggest Christmas Eve snowfall and deepest Christmas Day snow cover on record occurred on December 24-25, 1966, when 12.7" of snow fell, with 12.4" coming on Christmas Eve. The greatest Christmas Day snowfall was 5.5" as part of a 21.0" snowfall which fell on December 25-26, 1909.

How common a White Christmas is in Philadelphia would depend on how one defines it. According to The Philadelphia Area Weather Book:

Although snow has fallen in Philadelphia on Christmas Day in about one of every four years, about two-thirds of those sightings amounted to only a trace--that is, not enough to be measured. If a "White Christmas" is defined as at least an inch of snow that falls on Christmas Day, the frequency lessens considerably, to only about one out of every twenty-five years. On the other hand, if we loosen the criteria so that simply having an inch of snow on the ground qualifies, the frequency is about once every eight years. Whatever the definition, Christmas Day snow in Philadelphia is not very common.

The National Weather Service has a Christmas snow summary for Philadelphia from 1951 through the present.

A video taken on a White Christmas in 2009 is here.