To enjoy the best weather, avoid the largest crowds, and be assured that all seasonal attractions are open, visit Philadelphia [1] in May–June or September–October.
If you don’t mind longer lines for popular attractions and rising hotel rates, the city comes alive in summer with festivals, fireworks, and parades— especially during the week of Independence Day.
And if you don’t mind bundling up, winter offers the lowest hotel rates, fewest crowds, and if you’re lucky, perhaps a beautiful snowfall.
Links:
[1] http://www.moon.com/destinations/philadelphia/discover-philadelphia