Describe what happens when Gawain meets

the Green Knight on New Year's Day.

Gawain decides to take the place of his king in the challenge.

Explanation:

On Ney Year's Day King Arthur held a celebration at his castle when suddenly a green knight appears and challenges the king to a wicked game. In that moment the youngest of the knights of the round table, Gawain, offers himself to take the place of the king in the game.