Jordan Baker recounts that the night before Daisy's wedding she found Daisy with "a bottle of Sauterne in one hand and a letter in another." In addition, she explains that after reading this letter, the drunken Daisy tried to remove the pearls given to her as a gift telling Jordan to "Take 'em down-stairs and give 'em back to whoever they belong to. Tell 'em all Daisy's change her mine."
Jordan never confirmed the letter was from Gatsby, but safely assumed it was from him considering he had been with Daisy the entire summer before he left for the war. Jordan put all the pieces together when Nick mentions Gatsby at the beginning of the novel and then Gatsby himself tells Jordan of his affair with Daisy.
This detail influences the novel's plot in that it makes a lot of sense why Daisy is so willing to throw herself into the affair with Gatsby and why Jordan is so willing to help her. In fact, Jordan tells Nick that "Daisy ought to have something in her life" referring to Gatsby. It's important to note that without Jordan's relationship with Nick and her knowledge of this letter that perhaps the afternoon tea Nick arranges in the next few days doesn't happen. If that tea doesn't happen, perhaps this entire novel doesn't happen.
When Daisy was a young girl, she fell in love with Jay Gatsby. They were young and he was leaving for the war. They did not stay together and she marries Tom.
Before her wedding, she receives a letter from Gatsby. We do not know what was in that letter. It must have been something that left a profound effect on Daisy. We know that Daisy is not a drinker, yet on the eve of her wedding she gets drunks after reading the letter. It is sad, because although she deeply loved Gatsby, she marries Tom and has remained faithful to him. Tom on the other hand has not remained faithful to Daisy. Gatsby wants Jordan to persuade Nick to set up a meeting between Daisy and Gatsby. This is the turning point of the whole novel. When the two come together, we see that the love is still there for both of them. The tragic outcome that follows in the book all leads back to the meeting of Gatsby and Daisy.
The love of two people is a very strong thing. It is not always possible to just turn off those kinds of feelings. When you bring other people into the relationship, you set yourself up for failure. When there are spouses involved it is almost impossible for it to end in a good light. The letter that Gatsby wrote to Daisy, must have been extremely heartbreaking. She went and got drunk, which is not in her character. You can almost feel the heart wrenching pain that she felt when you read this book. Though what was in the letter remains a mystery, it is almost not important what it said. Daisy's reaction to it speaks loudly.