When the camp is about to travel, Aaron and his sons are to come and take down the veil of the curtain and cover the Ark of the Testimony with it.

Now you and your sons with you are to maintain your priestly duties and all matters that concern the altar and what is housed within the veil. You are to perform these services. I'm giving you the priesthood as a gift of service, but any unauthorized person who approaches is to be put to death."