He would celebrate a festive-day for his coming out from the Holy of Holies in piece.
Some say the High Priest (Kohen Gadol) made the feast at night and others say it was made on the following day. What is the basis for their views?
There are two reasons for this festivity:
1) When a person is in a dangerous situation and emerges from it safely, it is incumbent on him to make a thanksgiving offering (a karban todah). Thus, the High Priest made a feast to express his thanks to G-d that unlike others who were unworthy of the position or may have deviated in the incense offering and who would not come out alive from the Holy of Holies, he survived.
2) Because his service of the day was accepted by G-d. This was proven by the fact that he came out in peace.
If the reason is that his services were received by G-d, it makes sense that the feast should be at night, immediately following the service of the day. However, according to the view that it was a personal expression of thanks to G-d and was in lieu of a thanksgiving offering he intentionally waited till the morning because a sacrifice can only be offered during the day and not at night.1