Charles Ellicott Commentary Numbers 9:21

"And sometimes the cloud was from evening until morning; and when the cloud was taken up in the morning, they journeyed: or [if it continued] by day and by night, when the cloud was taken up, they journeyed." — Numbers 9:21 (ASV)

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 .