The best day of our protagonist's life happened when he was five and almost died at Disney World. Bummer about the early climax, dude.
Now he's sixteen, and generally pretty dissatisfied with what life has to offer.
On that best day, though, he was on the "It's a Small World" ride at Disney, and either heatstroke or his mother's love of Greek/Roman/Viking mythology stories at bedtime had him believing that the ride was really the afterlife.
In the midst of his panic about his whole family headed for certain death on the River Styx, he sees a little door behind the Eskimos and decides if he can just get to that door he can stop the ride and save their lives. Only trouble is he jumps out of the boat despite not knowing how to swim. #Fail.
After his dad rescues him and performs CPR by the igloos, the hospital proclaims him okay and his family goes back to the park, where everyone's so happy he's not dead that he gets to ride all the rides without waiting in any lines. Hence the whole "best day of his life" thing.