And Aaron and his sons you shall bring unto the door of the tabernacle of meeting, and shall wash them with water.
Read Chapter 29
Cyril of Jerusalem
The high priest washes himself, then offers incense; for Aaron was first washed, then became high priest. For how could one who had not yet been cleansed by water pray for others? Further, the laver had been set within the tabernacle, as a symbol of baptism.
Washed. The pagans never approached their mysteries, without divers purifications and washing. (St. Clement of Alexandria, strom. 5.) Exterior cleanliness was designed to signify the purity of the heart, with which we must appear before God. (Calmet)
It is for this reason we take holy-water, when we go into our chapels, and we wash our fingers before and during Mass. (Haydock)