Yoma, Chapter Three, Mishnah Three
This mishnah teaches a general principle with regard to immersing before service in the Temple.
1) A man may not enter the Temple courtyard or to worship even if he was clean until he immerses himself.
2) Five immersions and ten sanctifications did the high priest perform on that day.
3) And all in sanctity in the Bet Haparvah with the exception of this one alone.
Section one: There are actually two requirements in this section. The first is that one may not enter into the Temple courtyard unless he first immerses himself in a mikveh. The second is that one who is already in the courtyard may not begin to perform the Temple service until he immerses himself.
Section two: The high priest will change his clothing from his normal priestly wear to special golden clothes and then back again five times during Yom Kippur. For each change he would need to sanctify his hands and feet by washing them, making a total of ten hand and feet-washings.
Section three: All of these washings were done in an immersion pool which was on top of a chamber called the Bet Haparvah. The Bet Haparvah was within the Temple courtyard. The one exception was the first immersion which we learned about yesterday. Since this immersion was done before he began the service it was done outside of the Temple courtyard.