These are known, by the ancient Jewish sages, as the 3 days of darkness. Something terrible happened on Tevet 9....but no-one knows what.
These days of Tevet are December 23/ 24/ 25 of this year. I realise this will only be read on December 27th...and I am writing this on December 22nd. Will something have happened on those days, I wonder?
Rabbi G's very interesting code in the link.