In - (or at) the beginning of the watches “At the beginning of each night-watch” means all the night through. The Hebrews divided the night into three watches.
Related Commentaries of Lamentations 2:19
Lamentations 2:19
19 Arise, cry out in the night: in the beginning of the watches pour out thine heart like water before the face of the Lord: lift up thy hands toward him for the life of thy young children, that faint for hunger in the top of every street.