The whole week he could eat whatever he wants. However, right before Yom Kippur, they would not let him eat too much as he might fall asleep during the night.

Yuma 1:5

The elders of the cohanim would bring him to the Avtinas, upper chamber. There they made him swear before leaving him.

They told him ’My prominent Cohein Gadol, we are the messengers of Bais Din and you are our messenger. We make you swear by he who put his name on this house that you will not deviate from all that we have taught you’. Each of them would turn away and start crying that Cohein Gadol was suspected of not doing the right avoda.