Just when Yom Kippur began has been hotly debated by academics for over a century. The main question is whether it happened during the First Temple period. The evidence seems to indicate that it did not exist then.Again, read the article soon, before it goes behind the subscription paywall.
Writing just after the First Temple was destroyed by the Babylonians, Ezekiel seems unaware of Yom Kippur. It is not on his list of holidays to be observed when the Temple would be rebuilt.
A rare relic from the First Temple period (found in the City of David, referring to Bethlehem). Photo by: Clara Amit, IAA
Neither does Zecharia seem to have any notion of it when he instructed the Jews returning from captivity on observation of fast days. When Ezra reads the Torah to the returning Jews on the first of Tishrei, they learned that they need to prepare for Sukkot, but Yom Kippur is not mentioned. This is only proof of omission, but it’s all we have.
Thus, it seems that the three biblical mentions of the Day of Atonement (Numbers 29:7-11, Leviticus 16:1-34, and Leviticus 23:26-32) were inserted by priests during the Second Temple period to validate new rites added to purify the Temple in advance of the most important holiday in the Jewish calendar at the time, Sukkot.
The priests of the Jerusalem Temple who inaugurated Yom Kippur seem to have had the 12-day Babylonian festival marking the new year, Akitu, in mind, particularly the fifth day of Akitu, which has some striking similarities to Yom Kippur that are unlikely to be coincidence.
That fifth day involved a purification ceremony called kuppuru, which involved dragging a dead ram through the temple, supposedly purifying it of impurities. Kuppuru and its Hebrew cognate kippur meant “to uncover” or specifically in this case “to remove impurity,” which means a better translation of Yom Kippur to English would be "Day of Purification."
The root meaning of kippur may have been "to cover over," rather than "to uncover," but in any case its usage in the Hebrew Bible is "to expiate" or "atone for" sin. This is a somewhat different issue from removing impurity, which potentially involves ritual impurity, which is defiling but not sinful. The concepts of sin/atonement and impurity/purification do have some overlap in biblical Hebrew, but they are generally distinct ideas. I would stick with the translation "Day of Atonement."
For lots more on the Akitu Festival, see yesterday's post here and links, notably here. As I explain in the latter post, I don't see a direct connection between the Akitu Festival and Rosh HaShanah or Yom Kippur, but I do think it's possible that both descend from an indigenously Judean re-enthronement festival that operated on similar lines.
This article also discusses the scapegoat ceremony, on which see also here, and the Yom Kippur observances by the High Priest in the Second Temple period according to the Mishnah. For Yom Kippur in the Talmud, see here. For more on the mysterious figure Azazel in the biblical Yom Kippur rites, see here.