Today is the 9th of Tevet. Rabbi Dr. Shnayer Leiman has given one of the best shiurim I have ever heard on reason for the fast on this day (which nobody fasts on).
You can hear the shiur (or download it) on the Yeshiva University website:
The shiur begins with a cryptic Rashi in Avoda Zara which says that the Romans have no langugage or books of their own. He then looks at the Shulchan Aruch who says that 9th Tevet is a Fast Day, but we do not know the reason why. He cites attempted answers from Sefer Hakabballah, Rema (in Mechir Yayin), Taz and Magen Avraham, Ye'arot Devash and Sefer Ha-Ibbur (which claims that 25th December in the year of Jesus' birth fell on 9th Tevet). Then he gives what seems to be the most likely answer - that it is the Yarzheit of Saint Peter, who was really a Tanna named Shimon Kefas (Kasponi), uncle of Jesus. and he shows that the original text of rashi also made reference to this literature of the 'Jewish version of the Jesus story'.
I have never met Rabbi Leiman, and (unfortunately) have no connection to him, but I highly recommend this and all fo his shiurim.
In the shiur, he is reading from a source sheet. However, I was not able to find the source sheet on that website. So I created it myself (and the sources that he quotes are wild and wonderful).
So, if you want to hear his shiur, and read a source sheet, you can download this source sheet, and click on his link.