The best time to book is when you see a rate you can live with. You should be checking as far out as you can. Check daily if you have to. Prices change quickly. Right now PHL should have some good fares because Southwest came to town.
Check the website of the airline you want to fly as well as Travelocity, Expedia, Orbitz for ticket pricing. Many times you will find that the airline prices will beat all of these sites.