-Some of Old Testament Judaism's holidays are festive, but not Yom Kippur. It's not a day for celebration. It's a day to be unhappy; viz: a day to afflict oneself; which Webster's defines as causing distress so severely as to cause persistent suffering and/or anguish. (Lev 16:29, Lev 16:31, Lev 23:27, and Lev 23:32)
It is both illegal and curse-worthy for a Jew to be joyous at any time during Yom Kippur.
†. Lev 23:29 . . For whatsoever soul it be that shall not be afflicted in that same day, he shall be cut off from among his people
†. Deut 27:26 . . Cursed is the man who does not uphold the words of this law by carrying them out.
Note the grammatical tense of the curse. It's present rather than future indicating that the curse is immediate-- no delay and no waiting period.
I tend to suspect that curses are cumulative. In other words: every year that a Jew fails to afflict their soul on Yom Kippur, they add another curse to the indictment against them.
Why is Yom Kippur such a depressing day? Well; that's not too hard to figure out.
There's a goat involved in Yom Kippur commonly called a scapegoat. However, it's not really a scapegoat, it's actually an escaping goat; viz: a fugitive.
Once a year every sin the people ever committed is ceremoniously placed on the goat and it's then turned loose. Well; that right there is a very bad thing for Jews because in order for their sins to be pardoned, it's necessary for the sin-bearing goat to be put to death and burned on the Altar. So what? Well; I'll tell you so what. It means that the people's sins go unpunished; viz: the sins placed on the goat are still on the books hanging over the people's heads like a sword of Damocles and one day will be brought to justice. Now if that isn't reason enough to be depressed, then I give up.
Q: Why every sin the people ever committed?
A: Because there is not even one offering in the entire Levitical catalogue that suffices to delete the people's sins. The only thing that those sacrifices obtain for them is a reprieve; which is a temporary delay, not a permanent fix. In other words: forgiveness in the Old Testament acquits no one; Old Testament forgiveness merely puts justice on hold. (Ex 34:7, Rom 3:25-26, Heb 10:1-4)