The officer then said, ‘Go out and see if it is time to slaughter the tamid [daybreak]’. If it was time they would call out ‘barkaei’. Mattisayu ben Shmuel said the officer would ask, ‘Is the whole east lit up as far as Hebron?’ and the cohein would say, ‘Yes’.

Yuma 3:2

They had to do this since once it happened that the moon’s light was so bright they thought the day had started.

They then took the cohein gadol to the mikvah.

There was a general rule that if the cohein had a bowel movement he needed to go the mikvah. If the cohein urinated he needed to wash his hands and feet again.