How does the weather on the day of the lunch play into events of the chapter?
That the day Nick and Gatsby agreed upon for Nick to invite Daisy to tea was "pouring rain" seems like a metaphor for Gatsby's feelings. The weather is as unsettled as his emotions are. He is impatient and desperate, assuming that Daisy will not come when it is not even time for her to arrive. Then, when Daisy does arrive, Gatsby runs around the house so that he could knock at the front door, where, when Nick opened it, he stood "in a puddle of water glaring tragically into [Nick's] eyes." All Gatsby's apprehension and nervousness and fear and anticipation figuratively storm inside him, just as it literally storms outside.
When Nick goes to wait outside so that they can speak privately, he says, "While the rain continued it had seemed like the murmur of their voices, rising and swelling a little now and then with gusts of emotion. But in the new silence I felt that silence had fallen within the house too." The weather settles and the sun comes out, as Gatsby and Daisy talk inside, and when Nick returns, Gatsby "literally glowed" with happiness. While it had rained, Daisy had cried, but now her voice is "full of aching, grieving beauty" and "unexpected joy." Thus, the stormy weather seems to symbolize both of their tumultuous emotional states, and then, when the sun comes out, it symbolizes a sort of new day for this old couple.