Pelayo discovers the old man lying facedown in the mud, unable to pick himself up because of the weight of these wings. Márquez is a well known magical realist writer whose approach to literature is to drop elements of the extraordinary into otherwise realistic worlds.

The second sentence is also highly characteristic of magical realist writing, in that it imbues a non-human entity ("the world") with the personality traits and emotions of a human ("was sad"). This is known as personification, and it is a common approach within this genre, which frequently views objects and non-living entities as having agency or "lives" of their own.

Although the title of Gabriel Garcia Marquez’s short story “A Very Old Man with Enormous Wings: A Tale for Children” immediately alerts the reader the story will be a fantasy, the question asks at what point in the story is the reader aware it will contain elements of fantasy. The story begins with elements of realism: a storm, a sick child, a husband and wife along the shore, and angry storm. That changes when the protagonist, Pelayo, is returning to his home from throwing crabs back into the sea and comes upon a man-creature. Partially buried in the sand, he finds what looks to be an elderly man who has a pair of very large wings growing out of his back. Pelayo runs to get his wife so she can see the creature with her own eyes. After the couple’s attempts to speak with the man are met by a strange dialect, they assume he is a sailor from another county who was thrown off his ship, and they simply ignore his wings. It is not until one of their neighbors—an old, wise woman—declares he is an angel that the reader is lead into the world of fantasy.

“He is an angel,” she told them. “He must have been coming for the child, but the poor fellow is so old that the rain knocked him down.”

The statement “The world had been sad since last Tuesday” informs the reader the storm was long lasting, but there is no timeline set by the statement. The author personifies the world, giving it feelings and painting a picture of the world weeping with rain and dreariness. This statement affects how the reader views what happens throughout the rest of the story. It paints a universal picture of the state of the world and mankind.

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.