The day agreed upon was pouring rain.
One can view this as a supernatural extinguisher of Gatsby’s flaming passions for Daisy. Perhaps after finally seeing her in person, his ideas about her will be checked with reality, and his passion will diminish.
Rain also contributes to the tense mood. In an enactment of the pathetic fallacy, the rain adds an element of tension to the whole scene. Note that the rain stops as soon as Gatsby and Daisy get comfortable with one another.
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