F. Scott Fitzgerald uses two contrasting seasons to symbolise the events which occur in the novel: Summer and Autumn (Fall). The novel has its opening in early summer - a period of regeneration and new beginnings. Summer is associated with life and a lust for the enjoyment thereof.
This is exactly what Nick Carraway wishes for himself, to start a new life in New York. More importantly, it also signals a new life for James Gatz, or Jay Gatsby. Jay has reinvented himself and is on a noble, fantastical mission to win over and regain his lost love, Daisy. Summer adequately fits the profile that Jay wants to create for himself. The season denotes joy, it is the season of love, summer holidays and summer fun. It is in this period that Jay throws his lavish, over-the-top parties with practically every who-knows-who attending. This is his desperate attempt to draw Daisy closer and to woo her once again.
However, the heat of summer can also be stifling and overwhelming. People tend to become literally hot under the collar during periods of intense heat, and so it is in the novel. We read about Daisy's little tiff with her husband, Tom Buchanan, about his purported affair. This is also the period when Tom breaks Myrtle's nose for daring to derisively call out Daisy's name.
Also, not only does Jay finally get to meet Daisy once again, but he is also involved in a nasty confrontation with Daisy's husband. In the New York heat, it is this confrontation that ultimately leads to the tragic climax of the story. It is the heat, literally and figuratively felt by the lead characters, that drive three of them to a most unfortunate denouement.
Throughout the Summer the characters are engaged in what we may call 'summer activities': Jay and Daisy flirting, constant trips to New York, Tom's continued affair with Myrtle, having parties at Tom's New York apartment, Jay's extravagant parties, etc. The only bleak note in all this is the sad plight of Myrtle's husband, George, who lives in desperate poverty, is treated with derision by Tom and cuckolded by his vivacious wife.
In the final chapters, Summer is replaced by Autumn, but this time the change is negative. It is the commencement of a bleak period. There is a chill in the air. The grass looks uncut, the air is less bright, leaves are falling from trees. All this foreshadows the tragedy to come. Myrtle suffers a most unfortunate and untimely death. George Wilson swears revenge and believes that Jay is the one responsible for his wife's death, the one she had had an affair with. He shoots Jay in his swimming pool and then commits suicide.
The parties have stopped. No one wishes to be associated with Jay. Tom and Daisy leave town. Only Nick, Jay's father, the postman, a few servants and 'Owl Eyes' attend his funeral. No one else turns up. A truly tragic ending.