> deity of the sun serapis, greco-egyptian deity of the sun first encountered at memphis, where his cult was celebrated in association with that of the sacred egyptian bull apis (who was called osorapis when deceased).  the ancient festival in rome was known as the saturnalia. the emperor aurelian established an official holiday called sol invicti, meaning unconquered sun in honor of the sun god. it was held dec. 24 and 25 and established dec. 25 as the official solstice. all the other religions that worshipped sun gods also took dec 25 as their fixed date for their festivals. mithras: in roman mythology, mithras was often celebrated around the time of the winter solstice, which falls close to december 25th. mithraism, a mystery religion that became popular in the roman empire, celebrated the rebirth of mithras on this date.  in 350, pope julius 1 decreed that the birth of jesus would be celebrated on the same date as all of the other solstice sun gods, dec 25. watch this but ignore everything but the bits related to the sun and its worship.   
United Kingdom