“Imagine you have one people who have Yom Kippur on one day, and one on another,” Adler says. “There will be arguments and fights. It seems the High Priest understood that Yom Kippur was on Sunday, let’s say, and the Qumranites were on a Monday, so he came down there on their day, perhaps with troops… he wanted them to break their observance. It could have been in Qumran, but it could have been somewhere else.”The interpretation of 1QPesher Habakkuk col. 11 to indicate that the Qumran sectarians observed Yom Kippur on a different day that the Temple priests is not new. It's been around as long as I can remember. But it is a plausible reading of the passage.
For more on the work of Yonatan Adler on the origins of Judaism, see here and links.
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