Yoma, Chapter One, Mishnah Six
The mishnah has now reached the night before Yom Kippur is to begin. As we learned in mishnah four, he had to stay up all night. Our mishnah teaches that in order to keep himself awake he would study Torah all night (this never seems to work for me, but Im not a high priest, not even a priest).
1) If he was a sage he would expound, and if not, the disciples of the sages would expound before him.
2) If he was familiar with reading [the Scriptures] he would read, if not they would read before him.
3) From what would they read before him? From Job, Ezra and Chronicles.
a) Zechariah ben Kvutal says: I have often read before him from Daniel.
Section one: If the high priest was a sage, then he would expound upon the Torah all night. But if he didnt know how to interpret the Torah, then disciples of the sages would do this in front of him, thereby helping to keep him awake.
Section two: If he could read the Tanakh, then he himself would read. But again, if he couldnt even read, then the disciples of the sages would read in front of him. Note again how the mishnah takes into consideration the possibility that the high priest would not even know how to read from the Bible. The Talmud again reminds us that in the Second Temple period high priests were often positions appointed by the king based on political considerations and were not given to the most knowledgeable of the priests.
Section three: The sections that they would read to him from were sections that were considered particularly interesting and that would help keep him awake. Job and Daniel are compelling stories. Daniel and Ezra are partly in Aramaic, which may have made them easier to comprehend for a high priest who didnt even understand Hebrew. I am not sure what the attraction of Chronicles is. Note that all four books are from Ktuvim, the last portion of the Hebrew Bible.