Our Gemara on Amud Aleph quotes the Mishna which states:

And the High Priest would make a feast and holiday for his loved ones and his friends when he emerged in peace from the Sanctuary.

We must ask, what indeed was this feast and Yom Tov all about? Was he truly and simply happy to have emerged safely? If we were to accept that, why is it worthy enough for the Mishna to mention it? We know that there is a standing principle that one offers thanks when emerging safely from a dangerous situation. See for example Gemara Berachos (54b) that states that emerging from four key dangers require Be Thanksgiving blessing: being physically ill, traveling through the wilderness, being in prison, and traveling on the sea. Perhaps you might argue that the obligation is only for those four, however Shulkhan Arukh (OH 219:9) based on a Rivash says that emerging from any dangerous event requires thanksgiving (although possibly without a full beracha using Hashem’s name, ibid 10 see various poskim op. Cit.) We might argue this Mishna proves that other life threatening situations obligate thanksgiving or we might say, since the Mishna only mentions a celebratory feast and not a blessing, to the contrary, it proves the point that no blessing is incumbent.

Ultimately though I would say this Mishna cannot be a proof to either position, because this is a danger brought about by a non-physical agency. Which, I suppose is the chiddush here, that one can and ought to celebrate emerging from a spiritual danger. The applications may not be practical as how many of us can claim that we were in a situation of danger caused by non-physical entities?  Would Rabbi Akiva, who was the only one to enter the “Pardes” and emerge unscathed (Chagigah 14b-15a) be obligated to offer a thanksgiving blessing or feast?  So it would seem.

The Sod Yesharim (Yom Kippur 47) has an interesting take on what the celebration was all about.  The Radziner Rebbe quotes a Yerushalmi that makes a play on words.  The verse states: “And no man shall be in the Tent of Meeting when the Cohen Godol enters to the Holy place. (Vayikra 16:17)”  Asks the Yerushalmi, is the Cohen Godol not also a man?  Indeed, the Cohen Godol at that time was more like an angel.  The Radziner explains, the verse means to say, nothing of human should be in the Holy of Holies, that is the Cohen Godol achieves a total divestment of his physical concerns. This concept also explains another phrase, used by the Mishna: והכהנים והעם העומדים בעזרה כשהיו שומעים את השם שיצא מפי כה"ג When the Cohanim and the people standing in the courtyard heard the Ineffable Name as it emanated from the Cohen Godol’s mouth…  The Arizal explains that it does not state, “When they heard the Cohen Godol say the Ineffable Name”, rather it says, “the Ineffable Name emanated”  That means that Shekhina spoke from the throat of the Cohen Godol without his conscious control.

The idea of all this being that the Cohon Godol who entered and emerged from the Holy of Holies celebrated that he achieved a supernatural state during his encounter.  His return to his loved ones was not merely escaping death in a sense that he could have been annihilated if he was not on the proper level, but it was also about his landing and returning safely to Earth.