Q. How come the Shulchan Aruch says we don’t know what happened on the 9 of Teveis, and if we don’t know what happened, why should it be declared a fast day at all?

A. Indeed Shulchan Aruch (O.H. 580: 2) counts as one of the days that misfortunes occurred to our forefathers and it is proper to fast on them, the Ninth of Teves. The Mechaver adds; but we do not know what happened on that day.
Horav Shlomo Miller Shlit’a mentioned that the fact that we don’t know or we don’t remember today the historical reason why a decree or minhag was enacted, does not constitute grounds enough for not keeping it.
However, Mishna Berura (ibid. 13) indicates that in the selichos recited on this day, it is mentioned that the day marks the yohrtzait of Ezra Hasofer. Why we would observe the demise-anniversary of Ezra more than so many others we do not (such as the Avos or Aharon Hacohen) could be explained by what Tosafos Chadoshim on Megilas Taanis elucidates. This day actually marks the birth of “Oisso Hoish” and it coincides more or less with the winter solstice. Traditionally it was the occasion of pagan festivities such as Saturnalia. Later on, the Church, as it often did, adopted it and made it coincide with the birth in Betlechem of Yeshu and the celebration of Christmas, better known in our tradition as the night of “Nitel.” (From the term for being-born in Latin)
Since the event, turned out to be catastrophic to our nation, giving rise to all kinds of severe persecutions, inquisitions and many a holocaust, it was therefore established as a day of mourning and fasting. The fact that the reason was hidden and deemed forgotten or was disguised by Ezra’s yortzait, was simply to avoid further conflict with the Church, that would only increase their antisemitism and abuse.
This day follows the Eight of Teves, also a time for fasting mentioned in Shulchan Aruch (ibid.). It marks the day the Torah was translated into Greek. This gave rise to false and mendacious interpretations of the text, and thus gave authority and validity to the New Testament.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a.