Charles Ellicott Commentary Numbers 4:5

"when the camp setteth forward, Aaron shall go in, and his sons, and they shall take down the veil of the screen, and cover the ark of the testimony with it," — Numbers 4:5 (ASV)

Aaron shall come, and his sons ... — Under ordinary circumstances the high priest himself might only enter the most holy place on one day in the year. At the time of the moving of the camp, however, the Divine Presence seems to have departed from the Holy of Holies, and to have ascended in the cloud which gave the signal for the removal.

The covering vail. — Better, the vail of the hanging or curtain —namely, that which separated the Holy of Holies from the Holy Place. (Compare to Numbers 3:31.)