From FR Wiki:
"Midwinter (also known as Deadwinter Day) was a festival to mark the midpoint of winter in the Calendar of Harptos. It occured on a special day between Hammer 30 and Alturiak 1. Amongst nobles and monarchs it was known as Midwinter and was traditionally used to make or renew alliances, although the common people called it Deadwinter Day, a reference to the cold and hard times that remained before the spring.
"On Midwinter Day the Red Fellowship of the Deity known as the Red Knight observes the Retreat. This solemn ceremony consists of an assembly wherein the clergy discuss the previous year's campaigns. Strategies are discussed, battles analyzed, and the accumulated lore integrated into the church's teachings.
The holiest day of the year for the Church of Shevarashis Midwinter Night, during which the Dark Court Slaughter is remembered. Inductions into the ranks of the clergy occur at this time. Each new cleric screams vows of vengeance into the night air and swears neither to laugh nor smile until the Spider Queen and her followers are no more."
Bunch of other FR deity stuff, but this is particularly interesting:
"In 1372 DR, Bane the Black Hand regained his divinity after being reborn out of Iyachtu Xvim like a moth from a cocoon on Midwinter Night. He took back the portfolio of fear and was elevated to a greater deity."
http://forgottenrealms.wikia.com/wiki/Midwinter
Not sure what the MacGuffin pictured night be...?
