THE GREEN KNIGHT - THE MIDNIGHT ARCHIVE

One Christmas holiday, young Sir Gawayn (Morgan Le Fey's son, which makes him King Arthur's and Queen Guinevere's nephew) strikes a deadly beheading game with a mysterious green knight (ten feet tall, green-haired, green-skinned, with green weapons and armour, riding an equally green-coated and green-maned horse). Within a year and a day, Gawayn has a rendezvous with Death at the mysterious Green Chapel (or Green Shrine). He can't refuse, because he has sworn an oath.

The next year, Sir Gawayn leaves home on the quest of his life. Freezing to death and exhausted in the wintry wilderness, he decides to rest at the Bertilaks', a childless couple living in a magnificent castle. There, Gawayn is well fed and given a warm, cozy canopy bed with eiderdown covers - but he also strikes a deal with the Bertilaks: the husband will give Gawayn what he has hunted in the woods, but he must give his host in return what his wife has given him in the castle. However, Lady Bertilak proves to be exceedingly frisky, exceedingly seductive. He can't say no to her for that would lead to breaking the laws of courtesy, but he can't say yes either or he'll lead her to infidelity: a real catch 22. And there are only three days left till the fatal rendezvous!

https://en.wikipedia.org/wiki/Sir_Gawain_and_the_Green_Knight

In the version I grew up with, it was a Green GARTER. And this is the origin story of the Order of the Garter and its motto: "Honi soit qui mal y pense," shame on those who think ill (of wearing a women's garment).