http://steamtreasurehunt.wikia.com/wiki/Winter_Objectives (http://steamtreasurehunt.wikia.com/wiki/Winter_Objectives)
Scroll down to see all the upcoming challenges, and buy the games before it so you can surely get the challenge.
Hope I helped with this. (And sorry if someone has already posted this, didn't really look around)
Grand Prize = Every single game on steam
Dear god...