Day is the third book in a trilogy, and so the meaning comes in part from the sequence.
Night tells the story of the Holocaust itself, and of the Jews being taken to the camps and their time in the camps.
Dawn, a novel, chronicles the aftermath of the Holocaust as one (fictional) survivor experienced it.
Day is the third book. In it a fictional character who shares some characteristics with Wiesel reflects on what he went through and the meaning of the Holocaust.

Taken together, they indicate a passage from darkness to light, from tragedy to relative recovery. A day would be a new start.