One of the most distinctive characteristics of The God of Small Things is the way it jumbles time. We're actually farther ahead in the future on the first page of the book, which plunks us down in 1993, than we are at the end of the book, which shows us a scene in 1969. What's interesting about the ending is that even though it's the end of the book, it's not actually the end of the story. In fact, the end of the book places us at the beginning of things, so to speak.
Ammu has just left the dinner table on the day of Sophie Mol's arrival and goes outside to listen to the radio and have some alone time. She thinks of Velutha and walks out to the river, part of her expecting him to be there waiting for her. She doesn't see him, but he is there, floating in the water. They end up making love right there on the riverbank, and we learn that they continue to do so every day for the next thirteen days.
None of the drama surrounding Sophie Mol's death has happened yet. As a result, the end of the novel seems suspended in time. We already know, having read the rest of the book, that the relationship between Ammu and Velutha will lead to their downfall. Here, however, we see a version of their love that isn't tainted by blame or shame; it's just about the two of them, living their lives day by day and step by step. The novel ends with Ammu saying to Velutha, "Tomorrow" (21.82), because that's the only thing she can guarantee him. We've spent the whole book knowing what's going to happen and trying to figure out how it happened, so in a way, the ending of the book is the last piece of the puzzle.