Numbers 3 - 4

Aaron's sons are named and their commissioning to the priesthood is recorded.  They would stand for the firstborn of the nation that God commanded were consecrated to His service due to His deliverance of them from Egypt on the night that the firstborn were slaughtered by the Angel.  The 2,750 sons of Kohath were responsible for the care and transportation of the holy items in the tabernacle  The 2,630 sons of Gershon were responsible for the care and transport of the curtains and coverings of the tabernacle structure, while the 3,200 sons of Merari cared and transported the hardware used to erect the tabernacle.