Yoma, Chapter Three, Mishnah Two
The first half of this mishnah is a continuation of yesterdays mishnah where we learned how cautious the priests were to ensure that dawn had truly broken before they slaughtered the morning tamid sacrifice.
The second half of the mishnah proceeds with the descriptions of the Temple service.
1) And why was all that necessary?
a) Because once the light of the moon rose and they thought that the east was lit up and slaughtered the continual offering, [and afterwards] they had to take it out to the place of burning.
2) They led the high priest down to the place of immersion.
3) This was the rule in the Temple: whoever covers his feet required an immersion, and whoever passed water required sanctification [by washing] his hands and feet.
Section one: The reason for all of the strict procedures which we learned in yesterdays mishnah was that one time it happened that the moonlight was particularly strong and the priests thought that it was dawn. They then sacrificed the morning tamid before they realized their error. Since the tamid was slaughtered at night it was unfit and had to be burned at the place of burning where they would burn unfit sacrifices. They would then have been liable to offer another tamid sacrifice.
Section two: On Yom Kippur the before they offered the tamid they would lead the high priest down to immerse himself, a requirement of which we will read more in mishnah three.
Section three: Covers his feet is a euphemism for defecating (See Judges 3:24). A priest who defecates must fully immerse himself before serving again in the Temple. However, urinating only requires him to sanctify himself by washing his hands and feet and not his whole body.