Yoma, Chapter One, Mishnah Two
This mishnah teaches that during the seven days before Yom Kippur the high priest performs regularly duties which he may or may not perform during the rest of the year.
1) All seven days he sprinkles the blood and burns the incense and cleans lamps and offers the head and the leg;
2) And on all other days if he wants he offers, for the high priest is first in offering a portion and has first place in taking a portion.
Section one: Sprinkling the blood refers to the blood of the daily tamid offerings which are offered in the morning and the evening. The incense is mentioned in Exodus 30:1-8. It is offered daily on the golden altar. Cleaning the lamps means cleaning them from the accumulated ashes, waste-oil and putting in them new oil and wicks. This task also includes lighting the new flame. Finally, the head and the leg are the partsof the two daily tamid offerings which were put first onto the altar.
The point of this mishnah is that parts of the service and other Temple duties that he could take part in during the rest of they year, but did not have to if he did not want to, become obligatory for him during the week before Yom Kippur. This is so that he becomes more accustomed to working in the Temple. In other words, this is a warm up.
Section two: During the rest of the year, he need perform these tasks only if he wishes to do so. The mishnah points out that any part of the sacrificial service that the high priest wishes to perform he may do so. His right trumps that of any ordinary priest, even those whose shifts are currently serving in the Temple. He also is the first to choose whichever part of the sacrifice he wishes, even if he wasnt the one to offer it. As they say, its good to be the high priest!