Philadelphia is great to visit any time, although given the city's seasonal popularity and the constant flow of conventions, you'll find the best deals in the fall and winter. Concert and museum seasons run from early October to early June, and July 4th draws a festive crowd to Independence Hall.
The city has four distinct seasons with temperatures ranging from the 90s (30s Celsius) in summer to the 20s (around 0°C) in winter. (Below-zero temperatures normally hit only one out of every four winters.) Summers, the height of tourist season, can get swelteringly humid. In the fall, the weather becomes drier. Spring temperatures are variable; count on comfortable breezes. I like late September and late May best.
