At what point in "A Very Old Man with Enormous Wings" did you first know that it included elements of fantasy? Discuss the effect of the second...

Well, the title is a dead giveaway that something unusual is going to happen in this story. The sentence you've asked about, "The world had been sad since Tuesday," is an almost poetic way of saying that it has been raining for a long time: "Sea and sky were a single ash-gray thing." It doesn't take long for the real fantasy to begin in the first paragraph with the discovery of the old man. From there on, the story becomes more and more of a fantasy, with a spider-woman and sunflowers popping out of sores and other things.

I'm not sure you can read anything more into the second sentence than a description of the miserable rain.