Chapter 10 is titled "The Perfect Day" for several reasons. The most literal reason is that Jesse spends the entire day at the art museum with the teacher he has a crush on, Miss Edmunds.
It is also, though he does not know it yet, the day that his best friend, Leslie, drowns in her attempt to cross to the river to their imaginary land, Terabithia. The accident is made more tragic by Jesse's guilty conscience: he could have invited Leslie to join them in their trip to the museum, but chose instead to keep Miss Edmunds to himself.
On one hand, the day truly is "perfect" because he spent it in blissful ignorance of the loss he would face when he returned home, marking the end of his innocent childhood. But this also makes the chapter's title ironic, since this "perfect day" ultimately becomes the worst day of his life.