Yoma, Chapter 1, Mishnah 3. We continue with the seven days of separation during which the Kohen Gadol must withdraw from his home before Yom Kippur. Our Mishnah teaches what was done with him during those seven days, and how he was prepared for the service of the day.
Handing the Kohen Gadol over to the elders:
"Masru lo zekeinim miziknei beis din" - during these seven days, elders from among the elders of the court, the sages of the Sanhedrin, were placed at his disposal.
"V'korim lefanav b'seder hayom" - they would read before him the order of the day's service from the portion of Acharei Mos, which describes the service to be performed on Yom Kippur, so that he would be fluent in it.
"V'omrim lo: Ishi kohen gadol, kra atah b'ficha, shema shachachta o shema lo lamadta":
"Ishi kohen gadol" - a title and expression of honor.
"Kra atah b'ficha" - read the portion yourself, with your own mouth and lips.
"Shema shachachta o shema lo lamadta" - perhaps you have forgotten the portion, and perhaps you never learned it at all.
It should be remembered that we are speaking of the period of the Second Temple, when most of the High Priests were unlearned and simply purchased the position with money. Hence the concern that the Kohen Gadol was not fluent in the order of the service, and hence the need to read the portion before him and to demand that he read it himself.
Erev Yom Kippur:
"Erev Yom HaKippurim b'shacharis ma'amidin oso b'sha'ar hamizrach" - on the day before Yom Kippur, at the time of the morning service, they would station him at the eastern gate.
"U'ma'avirin lefanav parim v'eilim u'kevasim" - they would pass before him the animals that would be used in the service: bulls, rams and sheep, "kedei shehei makir v'ragil ba'avodah" - so that he would be familiar and practiced with all the animals he would use in the course of the Yom Kippur service.
Why, then, did they not pass goats before him?
Because the goat sent to Azazel is the type that carries upon it all the sins of the Jewish people, and they were concerned that seeing it would cause the Kohen Gadol sadness and a bad feeling. Therefore the goats were not passed before him, while the other animals were.
In summary: in this Mishnah we learned about the training of the Kohen Gadol during the seven days of separation: the elders of the court read before him the order of the day from the portion of Acharei Mos and demand that he read it himself, out of concern that he may have forgotten it or never learned it - as was the state of the High Priests in the Second Temple period. On the day before Yom Kippur, at the morning service, they station him at the eastern gate and pass before him bulls, rams and sheep so that he will be familiar and practiced with the service, while goats are not passed before him, out of concern for the sadness caused by the goat that carries the sins of Israel.