Many longtime natives of Philly will suggest that the best time of the year to visit is: NEVER! Pick any day of the 365, and some Philly native will tell you how miserable it is. Too hot. Too humid. Too cold. Too rainy. Too foggy. Too sleety. Too snowy. Too dirty. Too smoggy.
Born and raised in the city, my opinion is not quite so harsh. However, on the cold, clammy November day in Philly, after I had deliberately missed my office retirement party the day before, my spouse and I were on our way to sunny, dry Arizona. Traitors to the City of Brotherly Love, we wanted to escape Philly to live out our golden years among the desert's cacti, coyotes and gila monsters.
Have we ever regretted leaving Philly and its oppressive humidity and lousy weather? Well, sure, just a bit. We don't miss the job slavery and daily commute through crowded, weather-befouled city traffic. We do miss friends and family, and even those few comfy days of the year when we could enjoy a Phillies game, a stroll through springtime Fairmount Park or a summer swim with the kids in the community pool. However, in Arizona, we can enjoy strolls and swims just about every day in the year. Of course, the climate can be a factor here, too. You don't want to do much summer strolling in the desert when daytime temperatures go are as high as 110 by 8 am.
Philly, unlike hot and hotter, dry and drier Arizona, has four very distinctive seasons: spring, summer, fall and winter. Springtime is probably the best time to visit. Although there is frequent rain, the temperatures and humidity are in the 70s and bearable. The rain brings out the city's beautiful trees and flowers, particularly in Fairmount Park and along the Schuylkill River banks. So if you're visiting, bring along an umbrella, even if the day starts out with sunshine.
Summer in Philly is another matter. Most tourists visit in July and August, especially to see the city's downtown historic sites and many entertainment venues. They should expect extremely sweaty days and nights, when the temperatures hit the high 80s and the humidity drips in with the soggy 90s. Air-conditioned buildings, theaters and restaurants help relieve some of the discomfort. I don't know how those guys could sit and sweat in Philly's July of 1776, in those heavy clothes and powdered wigs, and still come up with the Declaration of Independence.
Autumn in the Quaker City is spectacular, with the brilliant reds and yellows of the trees and very comfortable weather. Until the end of October and sometimes into early November, the daytime weather stays in the high 60s and low 70s, making strolling around town very pleasant. Of course, the high humidity is always there, and so is frequent rainfall. Keep that umbrella handy.
Forget wintertime in Philly, unless you have to live and work there. Real heavy blizzards happen, but only about once every two years, but bothersome snow and sleet are frequent visitors. The constant humidity makes a cold day of weather in the 30s seem much colder, especially if you have to walk through it on a windy winter day. While the first hours of a snowfall can turn the old city into a picture postcard of virgin white, within a few hours, it is all dirty gray slush and being spattered everywhere by traffic. Philly city services are usually less than adequate at times of snowfalls of a foot or more, and driving can be hazardous for days until the slush finally seeps away.
Seriously, if you take the right clothing, a handy umbrella and keep an eye on weather reports, visiting Philly can be pleasant any time of the year. The historic downtown area is relatively small, so even on heavy snow days you can hoof it to Independence Hall, the Liberty Bell, Ben Franklin's house, major hotels, museums, fine restaurants and all the other tourist sites. However, if you have a choice, plan your visit for early May or late September, when the weather and natural scenery are at their best behavior.