For me, the two highlights of the Philadelphia Museum of Art in terms of times to visit are Friday nights and Sundays.
Sundays are "pay what you wish" but you must pay at least a dollar so in essence you get to see a world-class art museum for a dollar (this does not include special shows such as the recent Dali exhibition).
Friday nights offer live jazz in the museum atrium where you can sit at a table drinking wine and eating delicious chocolate mousse (full dinners are also served but I have never partaken). In addition, you can walk around the lower galleries in the museum on Fridays which includes the Impressionist collection the museum is particularly strong in. Unfortunately, on Friday nights, the upper galleries which include some of the famous "rooms" of the museum are not open.
In addition to the Impressionist collection, the museum is also particularly strong in "rooms" which include a Japanese tea house which will make you swear you are actually outside, various temples, Napoleonic palace-type rooms.
In sum, the museum is a must-see for locals and for visitors to Philly, It's great for families and for dates or just with friends. And the cafe in the basement is not overly expensive (considering it is a museum) with fairly tasty soups and sandwiches.