Please only ask one question per post, but feel free to ask the other question in another post when you're able. The idea of hedonism is found throughout the entire novel, The Great Gatsby. Hedonism is the philosophy which celebrates pleasure as the only reality, the only important thing. Hedonists live to make themselves happy, generally with little or no concern for anyone else around them. In chapter three, we see hedonism in post-war America in all it's glory--though it's not a pretty sight.
Gatsby is hosting yet another of his extravagant parties, though it's the first one we've seen. The opening paragraphs describe all the grand food and music and clothing and transportation which come together to create this weekly event. The entire party is grossly extravagant and excessive, providing grossly extravagant pleasure to Gatsby's guests. There are a "corps of caterers," the food is "crowded" onto gigantic buffet tables, there is a "whole pit full" of musicians, the spread of alcohol is impressive (especially in a time of prohibition), and the cars "are parked five deep in the drive."
When the guests have arrived and the party begins, the "the halls and salons and veranda are gaudy with primary colors." The group is shallow, full of "introductions which are forgotten on the spot and enthusiastic meetings between women who never knew each other's names" (presumably because they are not the wives of the men they came to the party with and they may never see one another again). There is outrageous laughter which swells to outrageous levels as the party progresses. Wild dancing takes over the dance floor, fights break out, and guests get more drunk and disorderly as the night progresses. Toward the end of the chapter when cars are leaving and one car crashes into a wall, "a pale dangling individual stepped out of the wreck, pawing tentatively at the ground with a large uncertain dancing shoe." He is so drunk he thinks he's simply out of gas and insists on trying to keep driving, when in fact the entire wheel is gone from the car.
This is a night of wild and uncontrolled revelry, and those who attend are there simply to make themselves happy. That is evidenced by their adulterous pairings, their outrageous behavior, and their lack of concern for anyone else around them. In fact, "people were not invited--they went there." Most of them didn't even know who Gatsby was, and those who did didn't really know him.
They got into automobiles which bore them out to Long Island and somehow they ended up at Gatsby's door. Once there they were introduced by someone who knew Gatsby and after that they conducted themselves according to the rules of behavior associated with amusement parks.
This is a pleasure-seeking, hedonistic bunch. Many of them have the money to have, on their own, all the things they find at these parties; but it's the atmosphere of no restrictions or rules except for pleasure which draws them here.
It is glorious and extravagant while it lasts; then, on Mondays, a crew shows up to repair "the ravages" of the night before. The five crates of lemons and oranges delivered from the city on Friday are, on Monday, "left at his back door in a pyramid of pulpless halves."