The winter solstice is tonight at 10:49 pm. The solstice is the nadir of the solar year in the Northern Hemisphere.
It is the day with the shortest day length and the longest night. On a more positive note, the days begin getting longer starting tomorrow. Ancient people celebrated the winter solstice even back in Neolithic (Stone Age) times. The winter solstice was traditionally considered the beginning of each year.
Because one year is actually a fraction more than 365 days, early calendars would gradually build up errors amounting to several days, causing dates to drift against actual solar events.
It is likely that New Year's Day and also Christmas Day were chosen to correlate with the solstice, or are at least related to the time of the solstice.
But due to various changes to the calendar, we now have three days of note instead of just one.
