According to the Mishna (2a), a week prior to Yom Kippur the Kohen Gadol was isolated in an office in the Temple – the lishkat parhedrin – where he received training for the Yom Kippur service.
It was taught in a baraita that Rabbi Yehuda said: And was it called the Chamber of Parhedrin, the chamber for the annual royal appointees? Wasn’t it called the Chamber of Balvatei, the chamber for ministers and council heads?
Rather, initially, during the era of Shimon HaTzaddik and his colleagues, who were rewarded with long lives due to their righteousness, they would call it the Chamber of Balvatei, a term connoting significance, since it was a place designated for the High Priest. However, because people were giving money in order to be appointed to the High Priesthood, the position was filled by unworthy individuals. Due to their wickedness, they did not survive the year, and they were replaced every twelve months like the parhedrin who are replaced every twelve months. Therefore the chamber was called disparagingly the Chamber of Parhedrin. Since the High Priest was replaced every year, the new appointee would renovate the chamber to reflect his own more elaborate tastes.
The term parhedrin referred to a Roman official who was appointed to a position for a single year term. This was commonplace whether the individual was elected by the Senate or if he acquired the position by paying off the right people. Among the officials appointed by this method were those who were responsible for controlling prices on a variety of goods and services. It was not uncommon for people in this position to try to acquire significant wealth by collecting exorbitant taxes during their short terms, well beyond the amount prescribed by Roman law.
The baraita refers to a period during the Second Temple when the Kohen Gadol was appointed based on the amount paid to the person in charge; during that period a different person was appointed every year, leading to the comparison with the Roman official. According to Rashi, the need to appoint a new Kohen Gadol every year stemmed from the fact that such people, who aspired to a position for which they were not worthy, invariably died during the course of the year. The Rid explains that it was simply like the case of the Roman officials – the appointments were paid for only for a single year.
Some commentaries argue that it was not the Kohen Gadol who was replaced every year, but rather it was the office itself. Since the occupants of the position of Kohen Gadol were more interested in their honor than in the spiritual importance of the position, each of them tore down the office and rebuilt it to show off their wealth and position of authority.