In 1918, in the Soviet Union, January 31 under the Julian Calendar system was the last day of its use.

CNN: Saturday,

On the contrary, no feast day for Sol is mentioned on that day until 80 years later in the Calendar of 354 and, subsequently, in 362 by Julian the Apostate in his Oration to King Helios (the Sun).

FORBES: Christmas and the Adoption of the Pagan Solstice