That is, if the Steam service comes back online anytime soon. As of the time of this writing 2:30ish on December 25 the service has been down for a few hours and nobody is sure just when it will pop back up.
All we can suggest is to keep checking and hopefully you can grab your copy before the deal expires. However, knowing Valve, I wouldnt be surprised if they extended the deal or offered some other act of goodwill as an apology for the outage.
From all of us here at MEGATechNews, happy holidays!