If possible, plan your Katahdin trip to fall on a weekday. Unless you have reservations inside the park, you need to queue up at the gate to get in. Once they have let in a pre-determined number of cars headed to each area (Roaring Brook and Abol Bridge) they close the gate.
It would be a shame to get up there and not be allowed in the park. On busy weekends people talk about getting to the gate at 4:00 and being 15th in line. If you arrive at 7:00 you may be shut out. Weekdays are much less crowded.