A.D. VIII KAL. IAN. was the winter solstice of the Julian Calendar which was used until the 16th century.
Under the Julian calendar, winter solstice was celebrated on December 25th.
https://www.rmg.co.uk/discover/explore/when-winter-solstice
"Winter solstice is an important time for cultures across the globe. Under the old Julian Calendar, the winter solstice occurred on 25 December. With the introduction of the Gregorian calendar the solstice slipped to the 21st, but the Christian celebration of Jesus’s birth continued to be held on 25 December."
Edited by TigerMatt at 23:32:54 on 12/26/18