When is the specific day? The FAA will only book you a flight the day before your class starts, so as long as you're not graduating literally the day before your class start date you'll be fine. They'll give you a phone number and you'll book with their third-party travel agency. The agents have always been willing to accommodate any time/airport/airline preferences I've had, so it's not like they force you into the worst flight they can find.
Technically if you really needed to, you could book the flight yourself and take your chances getting reimbursed, however it's a risk. They'll only reimburse you what they would have paid using the government's preferred rate, so you could easily be out a few hundred bucks. A guy in my class did that and only got $140 back for a flight he paid $300 for.