Like Cornelius, I'd assumed it was only running until the 21st, until I checked their T&Cs:
I kind of thought the fine print indicated that the End of the World promo was good for the whole month of December, not just until 12/21. Is this for sure?
What is the Apocalyptic Rewards Offer?
A. Stay Gold and earn 100 Microsoft Points when you spend 2,000 Microsoft Points on eligible Xbox LIVE Marketplace content. Purchases must be made from December 1 – December 31, 2012 via the Xbox LIVE Marketplace...
What Xbox LIVE Marketplace purchases are eligible?
A. Any content purchased on the Xbox LIVE Marketplace here, excluding Microsoft Points and Xbox LIVE Gold subscriptions purchases, are eligible for the offer. Purchases must be made between December 1, 2012 00:01 UTC and December 31, 2012 23:59 UTC. A maximum of 100 Microsoft Points can be earned per Xbox LIVE Rewards member for the offer.
When will I receive my 100 Microsoft Points?
A. Your eligible Xbox LIVE Marketplace purchases will be calculated at the end of December. The 100 Microsoft Points will then be deposited by January 15, 2013. During the Offer Period, your Points will not show up under Pending.