On what day does the Green Knight arrive at King Arthur's Castle in the Pearl-Poet's Sir Gawain and the Green Knight?
The Green Knight shows up on New Year's Eve during the King's yearly celebration. The day, or evening rather, is significant for a number of reasons. First, it is the middle of winter, a season most commonly associated with death; and, it is a matter of death that the Green Knight so happens to appear. Next, the color that the intruder is sporting is one that suggests life or vegetation, something that is completely opposite of death. These two images together causes confusion along with the game that the knight wants to play with the king's court. New Year's Eve is also symbolic with putting away old things or habits in exchange for new ones. The Green Knight's challenge also asks for an exchange--a head for a head!