You should definitely book your flights and hotel sooner rather than later, especially around the holidays. You can always cancel a hotel and Southwest at least will allow you to change flights for no charge. Depending on when your SR goes through BS21 you may not know until a day or two before. In our case, my son was in a push division. He went through BS21 Wed night into Thur morning and then went straight into PIR practice. We didn't get the call until late Thursday afternoon when we were driving to the hotel from the airport!
And as soon as you have your travel plans in place, let your Recruit know that you're coming. It can be a great morale boost if he can look forward to your being there at PIR.
We already book our hotel, you can cancel your hotel 24 hrs before your date,,,Flights that a hard one... they say in the letter that they highly encourages you to purchase refundable airline in case your recruit does not graduate on time. we might drive,,, our sailor graduate on 1/8 ship 14 Div 056
Refundable airline tickets are ridiculously expensive. Instead shop around for the airlines with no penalties for rescheduling. Southwest is great in that regard.