Was Sigd part of historical Jewish observance?
I noticed that my (paper) calendar reports that 29 Cheshvan is "Sigd". Curious about this day (on the Jewish calendar; they style secular stuff like Election Day differently) that I'd never heard of, I did some searching. The Wikipedia page has some basic information and also several "dubious source" notations. From there I learned that this is a special day introduced (or reintroduced?) by the Beta Yisrael (Ethiopian Jews) and added as an Israeli national holiday in 2008. The page links to an article in the press that contains more background, including the claim (by leaders of the Beta Yisrael) that this day was traditionally observed by all Jews originally but then it fell out of practice:
“The Sigd is not something that the community created there [i.e., in Ethiopia]. It was observed here, in Jerusalem, after the return from Babylon,” according to Qes Emaha. “And so our community safeguarded it, in order to strengthen Judaism, and we kept it for 2,500 years.
Why the rest of the nation of Israel didn’t keep it, we don’t understand. We now want the entire nation of Israel to preserve this heritage: that it be a yom shabbaton [day of rest] for the entire nation of Israel. The heritage of the Beta Israel comes from the Torah and Judaism, and it is important to us that it will not vanish at some point. Just as the Sigd was not forgotten, so too the rest of the traditions of the Beta Israel must not be forgotten.”
What do we know of historical observance of Sigd? The Wikipedia article notes two oral traditions, one dating it to the 6th century CE and the other to the 15th, though the quote from the Beta Yisrael seems to make a much earlier claim. Does any rabbinic literature discuss this day?