For the most part after Labor Day and up untill Memorial Day the price out of EWR to STI, POP, and SDQ remain around the $400 mark (taxes and fees included), with the exception of major holidays.
I would buy the ticket anytime. The sooner you buy the closer towards the front of the plane your seat will be. Use the online seat selector. A little secret I learned is to print my boarding pass 24 hours in advance. But before you print the boarding pass review available seats. You will be surprised what seats become available 24 hours prior to departure.