The darkest night
During the Christian era people lost the exact date for the midwinter solstice. That was due to the need of a calendar. That calendar changed during time. With new religions (not only Christianity) our world had become flat. The festivities around the midwinter solstice were still important, but the date was no longer exact.
Among common people the 13th December became the darkest and longest night. Besides, the catholic name that day happened to be Lucia. Now, who the heck was Lucia? Sounded more like Lucifer… aha, probably the wife of Lucifer? Done. The darkest night, the most fearsome night of the year was the 13th, (lucky number?) and on top of that, carrying the name of the devil himself! Done.
This night was so long that you didn’t dare going to bed. You might get trapped in some dark time loop and never wake up again! Or God knows what demons could possess you during that night. Better stay awake. (And get another drink). And for heaven’s sake, don’t get out!