Here's how it is described:
From 26 December – 6 January, you can download a gift each day—songs, apps, books, movies, and more—with the 12 Days of Gifts app. Each day's gift will only be available for 24 hours, so download the free app to make sure you don’t miss out. Please note: Not all content is available in all countries.The app is free, and the gift is free. Basically, starting on Dec. 26 -- the day after Christmas, which makes us wonder why we don't get something "for Christmas" from Apple -- and running through Jan. 6, 2014, the app will gift a special free item from the iTunes, iBooks, and App Stores.
Each musical, video, e-book, or app gift is only available for 24 hours. Therefore, users have to remember to check the app daily to ensure they don't miss out on some nifty gift (or non-nifty gift, depending on their opinions).
Again, this is the first time the 12 Days of Gifts app has hit the United States, so U.S. users should be sure to pick up the app ASAP. You have plenty of time, as the first freebie won't hit until Dec. 26.
Then it's up to users to set an iOS appointment or reminder (which is not always the most reliable thing in the world) in order to check the app daily, starting on Dec. 26.