When does Elie Wiesel in the book Night get separated from his mother?

When the family arrives at Auschwitz, Elie and his father are separated from his mother in the sorting.

Although things were bad in transit, things get much worse when they arrive at the concentration camp.  The family and most of the other Jews have no idea where they are and what is happening. They have never heard of Auschwitz.

Right away, the men are separated from the women and children.  Since Elie is old enough...

When the family arrives at Auschwitz, Elie and his father are separated from his mother in the sorting.

Although things were bad in transit, things get much worse when they arrive at the concentration camp.  The family and most of the other Jews have no idea where they are and what is happening. They have never heard of Auschwitz.

Right away, the men are separated from the women and children.  Since Elie is old enough to work, he goes with his father and the men.  Everyone forms two lines, and the women and children are marched off.

And I walked on with my father, with the men. I didn't know that this was the moment in time and the place where I was leaving my mother and Tzipora forever. I kept walking, my father holding my hand. (Ch. 3)

Elie and his father’s line is further evaluated after the sorting by Aushwitz’s Angel of Death Dr. Mengele.  He wants to make sure that everyone is fit to work.  Those who are not are sent off to die in the crematorium.  This is Mengele’s job.

A man questions them first.  

The man interrogating me was an inmate. I could not see his face, but his voice was weary and warm. (Ch. 3)

Elie is told to say he is eighteen instead of fifteen, and his father is to say he is forty and not fifty.  In this way, they both will be seen as fit to work.  Elie won’t be a child, and his father will not be an old man.  This man saved their lives, and another inmate berates them for not getting away sooner.  They are stuck now.

The sorting is one of the most tragic events that befalls Elie and his father.  In one moment, his mother and little sister are sent to their deaths.  He has no way of knowing what is actually happening at the time, or how close to death he came himself.  This is the horror and inhumanity of Aushwitz, where Jews who were not useful for slave labor were immediately killed.  Even the ones who were saved would often die of overwork, exposure, disease, and starvation.