Today is the Winter Solstice in the Northern Hemisphere: the darkest day of the year.
Apparently, the exact time and date of the Winter Solstice this year in the Northern Hemisphere - as in, the astronomical event that occurs at the time we call the Winter Solstice - will be at 16:28 (that's 4:28pm) on Thursday 21st December 2017.
Of course, that's based on UK time, so you have to adjust that time depending on where exactly you happen to be in the Northern Hemisphere. If it helps with your calculations, this post is scheduled to go live 6 hours and 28 minutes before that time.
Anyway... Whether you celebrate/acknowledge it or not...
Blessed Winter Solstice!