Gatsby's party in chapter three expresses the Jazz Age in its sheer excess but captures the mood of the era in other ways as well. First, the party defies the Prohibition: drinks flow freely. There's a "bar with a real brass rail," writes Nick, "stocked with gins and liquors and cordials ...floating rounds of cocktails permeate the garden outside ..."
Second, the party depends on the availability of automobiles, another hallmark of the age: "the cars from New York," we learn, "are parked five deep in the drive." In a significant foreshadowing event at the end of the party, there's a car accident as the drunken revellers leave by the dozen, and the driver accused of the accident claims he wasn't driving the car.
Third, in allowing anyone entry, the party reflects the new openness of the Jazz Age, where people of different classes mingled freely. (This will later annoy the class-bound Tom Buchanan.) Gatsby perhaps exemplifies this exuberant freedom: much of the gossip at the party involves circulating wild stories about who he really might be, stories that in their contradictory excess seem to assure him his anonymity. This anonymity also reflects the age: who puts it better than Jordan Baker when she says, "I like large parties. They're so intimate. At small parties there isn't any privacy."
Further, as events at the party unfold, we gain some insights not only into the superficiality of the Jazz Age, but some hints into the flimsiness of Gatsby's self-construction: Owl Eyes notes that the pages of the books in the library have not been cut, though the books themselves are real, a fact that startles and impresses him.
It's at this party that Nick meets Gatsby for the first time and is charmed by his smile that "concentrated on you with an irresistible prejudice in your favor."
Alcohol, autos and anonymity as classes and sexes mix with a new freedom: all of these weave through the narrative of Gatsby's party.
Gatsby's party reflects the atmosphere of opulence and extravagance of the Jazz Age.
Characteristic of these parties is excessiveness: Every Friday five crates of oranges and lemons arrive at Gatsby's. Eight servants and an extra gardener set to work cleaning and shearing shrubs and repairing the ravages in the lawns. On the day of the party, there are caterers who arrive with hundreds of feet of canvas and colored lights that are plentiful enough to form a huge Christmas tree.
Machines in the kitchen extract juice from the crates of oranges and lemons brought in earlier. This juice is placed on the buffet tables that; these tables also hold hors d'oeuvre, spiced baked ham, turkey, and "pastry pigs." In a hall there is a bar with real brass rail at the foot; all sorts of liquor is stocked in this bar. The other halls are garishly decorated with bright colors. Outside, there is an orchestra of all brass instruments, and there is the chatter of voices and a "sea-change of faces and voices and color under constantly changing light."
The guests are not really invited; they simply show up, having heard of the lavish parties of a man who is not part of the established rich. They load into cars and drive out to Long Island and end up at Gatsby's. Afterwards, "they conducted themselves according to the rules of behavior associated with amusement parks."