Can I earn the Gangnam Style series, Where Is He? series, or Guide My Sleigh series hats on questions or answers that were posted before Winter Bash opened, or only on new posts? It isn't clear from the Winter Bash hat description page.
Hats can only be earned with activity performed after 19 December 2012 00:00 UTC. The exception to this is the L'Chaim hat, due to what it's celebrating and our own constraints.
Basically: get to posting! :D