In an unusual move, Philadelphia's Division of Aviation (DOA) includes WN flights on its latest timetable; which just came out and covers the 2nd quarter of the year (Apr. 1 - June 30). On the airline listings, it does state that WN flights will start on May 9. I believe it only lists the original 14 flights and not the additional 14 flights that will be added in June.
This is unusual because in most cases, the timetables are only updated to what's PRESENTLY on PHL's flight roster. I guess there's much to be said about advance notice and advertising plus DOA's commitment to inform passengers about WN flights.
These timetables are available at all airport service information counters (public side of the airport) and at SEPTA's Market East Regional Rail Station. I'm not sure if these timetables are available at Suburban Station or 30th Street Station. At the train station(s), the timetables are located where one picks up train schedules.
Feel free to pick up a copy.