Before a kohen enters the Temple to serve, the Torah commands him to “sanctify” (i.e., wash) his hands and feet, even if he is already tahor. The Sages further require that he immerse his entire body (in accordance with R. Yehuda in Yoma 30a; MT, Laws of the Yom Kippur Service 2:3). He need not wash or immerse again as long as he serves continuously. If he urinates, he must wash his hands and feet again. If he defecates or leaves the Temple environs for a significant amount of time, he must immerse his body and wash his hands and feet again (MT, Laws of Entering the Temple 5:3-5).
For immersing, there was a mikveh adjacent to the Temple courtyard (azara), and for washing the hands and feet water was used that came out of the faucets affixed to the kiyor (laver), located between the Sanctuary and the outer altar. The kohen would place his right hand on top of his right foot and wash them together, and then place his left hand atop his left foot and wash them together. A kohen who served in the Temple without first washing his hands and feet was liable to punishment by death at the hands of heaven, as we read:
Make a laver of copper and a stand of copper for it, for washing; and place it between the Ohel Mo’ed (Tent of Encounter) and the altar. Put water in it and let Aharon and his sons wash their hands and feet from it. When they enter the Ohel Mo’ed they shall wash with water, that they may not die; or when they approach the altar to serve, to turn into smoke an offering by fire to the Lord… (Shemot 30:18-20)
On Yom Kippur, in addition to immersing and washing before beginning the avoda, the Kohen Gadol also had to immerse each time he changed from the golden vestments into the white vestments, and vice versa, as we read, “They are sacred vestments; he shall bathe his body in water and then put them on” (Vayikra 16:4), and “Aharon shall go into the Ohel Mo’ed, take off the linen vestments that he put on when he entered the holy place, and leave them there. He shall bathe his body in water in the holy precinct and put on his vestments…” (ibid. 23-24). Additionally, it is a mitzva from the Torah for the Kohen Gadol to wash his hands and feet twice with each change of vestments: once before removing the old set and once after putting on the new. Thus, the Kohen Gadol immersed five times and washed his hands and feet ten times on Yom Kippur (Yoma 32a). During the year, he used the laver like the rest of the kohanim. On Yom Kippur, in his honor, the water was brought to him, in a golden ewer, so that he would not have to return repeatedly to the laver (Yoma 43b).
The idea behind immersing is to become pure and to extricate oneself from an existing state to rise to a new, more elevated state. The white vestments were of a higher rank in that they could bring one to a higher, more abstract plane, and the golden vestments were higher in that they revealed sanctity within the full variety of this world. In order to transition to new, higher states, as expressed in the changing of clothes, the Kohen Gadol had to immerse.
The sanctification (by washing) of the hands and feet was meant to elevate and unite all the energies of the kohanim in service of the divine. The focus was on the hands and feet because they express the actualization of a person’s potential – working with his hands and entrenching his work in the world with his feet. The ten fingers hint at this as well. They correspond to the ten statements with which God created the world (Avot 5:1). Human beings, created in the divine image, use their ten fingers to repair and improve the world.
On Yom Kippur, the Kohen Gadol had to sanctify his hands and feet twice with each change of clothing. The first time was before removing the vestments he had worn during the most recent avoda, because when a person is privileged to engage in especially holy and uplifting work, it invigorates him and amplifies all his drives, positive and negative. An intense divine light courses through him, protecting him from the amplified desires which might lead him astray. But when he finishes the work, those same desires may snare him. To protect him from them, the Kohen Gadol was commanded to wash his hands and feet before removing the garments. This allowed him to seal in all his energies with holiness, which he tapped into during the avoda. The second time he sanctified his hands and feet was after he put on the new garments. This was to purify all his energies prior to undertaking the next element of the avoda, and to ensure that they were not still preoccupied with his previous avoda (based on Orot Ha-teshuva 14:33).