On a normal visit to Adventureland, guests are allowed to leave and return throughout the day (or at least, were allowed to the last time I visited).
The Phantom Fall Fest has some specific restrictions to this. From their FAQ:
Guests may leave the park and re-enter the park prior to 6:00 p.m. on Saturdays and Sundays. Re-entry is not permitted on Fridays.
https://www.adventurelandresort.com/things-to-do/events/phantom-fall-fest/faqs
Plan accordingly. If you leave on Friday, you can’t come back (unless you buy a new ticket, I suppose). If you want to pop out of the park to go grab a bite to eat on Saturday or Sunday, do so before 6 p.m.