When Nick returns home, he parks the car and sits outside to enjoy the summer night in The Great Gatsby, by F. Scott Fitzgerald. What makes it such a beautiful night?
In chapter one of The Great Gatsby, by F. Scott Fitzgerald, Nick Carraway goes to visit his cousin Daisy and her husband Tom Buchanan on East Egg, New York. East Egg is the home of those with old money, the entitled and the careless, at least in the case of the Buchanans. It was a rather stressful visit for Nick, as he heard Tom waxing eloquent about the superiority of the white race (a completely racist viewpoint), he discovered that Tom is cheating on Daisy, and he learned that Daisy knows her husband is an adulterer but stays with him anyway.
So, when Nick drives back to his humble home on West Egg, he was undoubtedly enjoying the peacefulness of the moment. Specifically, this is what the novel says:
Already it was deep summer on roadhouse roofs and in front of wayside garages, where new red gas-pumps sat out in pools of light, and when I reached my estate at West Egg I ran the car under its shed and sat for a while on an abandoned grass roller in the yard. The wind had blown off, leaving a loud bright night with wings beating in the trees and a persistent organ sound as the full bellows of the earth blew the frogs full of life. The silhouette of a moving cat wavered across the moonlight and turning my head to watch it I saw that I was not alone--fifty feet away a figure had emerged from the shadow of my neighbor's mansion and was standing with his hands in his pockets regarding the silver pepper of the stars. Something in his leisurely movements and the secure position of his feet upon the lawn suggested that it was Mr. Gatsby himself, come out to determine what share was his of our local heavens.
Specifically, this is a beautiful night because the earth is making the sounds of summer (frogs, wind, trees) and the moon is shining above; it is a relatively quiet, peaceful place and time. Nick also sees Gatsby for the first time, and the man's distinctive and calm presence is a sharp contrast to the arrogant Tom and the languid Daisy and Jordan. Mostly, though, it is just a peaceful summer night.