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.
Chapter 10 describes the perfect day of Jess because that is the day he gets to visit an art gallery for the first time, and what is more, he gets to visit it with his favourite teacher that he is clearly infatuated with, Miss Edmunds. This sight of great works of art for the first time really inspires him: "he was drunk with colour and form and hugeness", as does the interest that Miss Edmunds takes in his artistic talent.
However, it is important to note the irony of the naming of this chapter, for it is at the end of Jess's "perfect day", that he finds out about Leslie's death. Perhaps his day wasn't so perfect after all.
Until he came home to the devasting news that his best friend had died, the rainy day described in Chapter 10 was perfect for Jess because he was invited on a trip to Washington, D.C. by his favorite teacher, he got to visit the nation's capital and a museum for the first time, his teacher bought him lunch in the cafeteria, and the sun was shining brightly as they returned home from the outing.
Jess has a crush on his favorite teacher, Miss Edmunds. Jess hardly ever gets phone calls, so when Miss Edmunds calls him to invite him to go to the city, he is so excited that his voice shakes and he breaks out in a cold sweat. The opportunity to go on an outing with Miss Edmunds is like a dream-come-true to Jess.
Miss Edmunds takes Jess to the National Gallery in Washington, D.C. Jess, whose experiences beyond the boundaries of his home have been very limited, has never been to an art gallery, nor has he been to the nation's capital, even though it is only a short distance from where he lives. Jess is surprised that the famous landmarks of the city look very much like "the way the books had pictured them", and he is awestruck at the majesty of the museum gallery, which to him is "like stepping inside the pine grove - the huge vaulted marble, the cool splash of the fountain, and the green growing all around".
Miss Edmunds takes Jess to eat lunch in the museum cafeteria, and insists on paying, which is a good thing because Jess has not brought any money. Jess enjoys a three-dollar meal, which is "far more than he had meant to have her spend on him".
As Jess leaves the museum with Miss Edmunds, the rainy sky has given way to "brilliant spring sunshine". The beautiful weather is representative of Jess's spirits as Miss Edmunds tells funny stories on the ride home from what has been a perfect day (Chapter 10).