Night is written as a memoir. This means that the book is written in the first person, relating the personal experiences of the author. Because of this, we can assume that the book is a reasonably accurate portrayal of the events described.
However, when a memoir is written, one of the problems that often appear is the fact that a long time might have passed between the events described and the writing of the text. One of the ways in which this problem would have been reduced is if Night was written as a diary. In this case, the events would have been described immediately after they happened.