liber pontificalis

Notably the first of the early popes not to be venerated as a saint in the Roman Rite (though fêted in the Eastern Orthodox Church on 27 August and on 4. Pi Kogi Enavot in the Coptic Church, one of the calendar’s epagomenal, “monthless” days), Liberius (*310 - †366) was Bishop of Rome from May of 352 until this death, on this day. Liberius was the first pontiff to associate the winter solstice—celebrated at the time as Sol Invictus—with Christmas, not only as means of co-opting a popular pagan holiday but also in line with the reasoning that great figures did not live life in fractions of years.  This date was also championed by his predecessor Pope Julius I and penned down in subsequent years.