In the higher, cooler elevations of Central Pennsylvania, bulbs may be planted after the middle of September up until Halloween. In Western Pennsylvania, hold off planting until Oct. 1, and in the Delaware Valley close to Philadelphia, wait until after Oct. 15.
Spring-flowering bulbs like daffodils and tulips are planted in fall. Garden centers and mail-order bulb companies also sell fall-flowering bulbs like colchicum and autumn crocus. These species are planted in September -- as soon as possible so they root and produce their flowers later in fall.
Contact your office of the Penn State Cooperative Extension Service for regionally specific recommendations for bulbs and planting schedules in your part of the Keystone State. Overall, October is better than September for planting bulbs.