By day or by night. — It is obvious from this verse that there must have been sentinels constantly watching by night as well as by day, whose office it was to give notice when the cloud was removed. (Comp. Psalms 134:1.)
Related Commentaries of Numbers 9:21
Numbers 9:21
21 And so it was, when the cloud abode from even unto the morning, and that the cloud was taken up in the morning, then they journeyed: whether it was by day or by night that the cloud was taken up, they journeyed.