What does the green knight propose?

The green knight demands, somewhat arrogantly, to speak to Arthur, while the court stares on in stunned silence. When Arthur finally speaks, the stranger explains that he has come to this famously valiant court to play a Christmas game. Whoever agrees to play this game will be allowed to strike the Green Knight on the spot, in the middle of the court; in exchange, the Green Knight will strike a return blow upon the volunteer a year and a day hence.