Jordan Baker is the one who tells Nick the story of how Jay Gatsby and Daisy met in The Great Gatsby by F. Scott Fitzgerald. We believe the story, despite the fact that Jordan is a liar and a cheater, because it sounds perfectly in character for both of them.
In October of 1917, when Daisy was eighteen and Jordan was sixteen, Daisy met Gatsby. She was a young, rich socialite (always dressed in white) and he was a soldier (dressed in his uniform). Jordan could see that Gatsby was completely in love with Daisy; however, when Daisy tried to go east and say goodbye to Gatsby, her family was not happy and forbade her to go.
Daisy comes home and pouts for a while, but by the next fall (October 1918) she was again acting the rich socialite, though now her friends were a bit older and more mature. By June of 1919, she is preparing to marry Tom Buchanan. This is all pretty straightforward information, and the only real speculation we have to make is whether she stopped loving Gatsby once he left or if she was just spending time with other men in order to forget him. Based on the next thing we learn about Daisy, it seems likely that she did, at least in her own way, love Jay Gatsby.
Jordan was one of Daisy's bridesmaids, and she tells Nick about how she discovered that Daisy probably still loved Gatsby.
I came into her room half an hour before the bridal dinner, and found her lying on her bed as lovely as the June night in her flowered dress-and as drunk as a monkey. she had a bottle of Sauterne in one hand and a letter in the other.
It was frightening moment for Jordan, as she had never seen Daisy this way. She tries to ask Daisy if she is okay, but Daisy is not listening.
"Here, deares'." She groped around in a wastebasket she had with her on the bed and pulled out the string of pearls. "Take 'em down-stairs and give 'em back to whoever they belong to. Tell 'em all Daisy's change' her mine. Say: 'Daisy's change' her mine!' " She began to cry--she cried and cried. I rushed out and found her mother's maid, and we locked the door and got her into a cold bath. She wouldn't let go of the letter. She took it into the tub with her and squeezed it up into a wet ball, and only let me leave it in the soap-dish when she saw that it was coming to pieces like snow.
From this story we gather that the letter was from Gatsby, and Daisy is a mess because she still loves him but knows she cannot (will not) marry him. She never drinks, but she drank here to try to cope with the reality that she is marrying someone other than the man she really loves. She wants to give everything back and deny her relationship with Tom, but of course Jordan and the maid cannot let her do such a scandalous thing. They sober her up and Daisy gladly marries Tom, as if the letter incident had never happened. Though she appears to be imminently happy, we do know that Tom was "fooling around" with a girl from the hotel on their honeymoon.
The reality is that Gatsby, dressed in uniform, did not readily show the difference in their social classes, but Daisy's parents understood perfectly what letting Daisy marry such a man would mean for all of them, and they were not about to let that happen. Daisy could of course have chosen love over money and social standing, but she did not. Instead she chose an arrogant and somewhat abusive man who she had to know would cheat on her.
The four years after that had to be miserable for Daisy, and then she hears that name again--Gatsby.