Longnight

Longnight is the winter solstice, the longest night of the year around the world.  This holiday is intercalary, meaning it does not belong to any particular month, and always takes place between the months of Spiritrest and Snowfall.  Spiritrest is a month full of post-harvest celebrations around the world, but no day (or night) is as big of a celebration as Longnight in as many places around the world.

Unusual Circumstances

On Longnight, even the stars and moon do not appear in the sky.  Even during the day, the sun appears somewhat blurry, shrouded or indistinct.  The weather on Longnight is always more mild than is seasonal.
There are rumors of even stranger patterns, though they are difficult to support with facts by their nature.  Common rumors include: A pregnancy will only occur if desired by both parties, you can drink twice as much as usual without experiencing a hangover, a fortune told on this night will always come true, any number of things will guarantee true love,  the gods are not watching the world as they are off doing something else, making an oath to a god will result in a blessing from that god that assists in maintaining the oath, it is easier to find or open portals to the Feywild or Shadowfel, and on and on.
All of this has lead to as many myths, legends and theories about what exactly is happening on Longnight that makes it so different.

Origins

Given how widespread the celebration of Longnight is, the origin of the celebration is almost certainly from well before the Divine War calamity, and may even be from before calamities prior.  Longnight seems to have been a celebration as far back as it is possible to find out, and the celestial circumstances seem to be the same since the celebrations began.  Most scholars on the subject agree that Longnight merely became a celebration around the world because the unusual characteristics during that day seem to be universal.