This promotion will be available until January 4, 2017. Once it’s attached to your Uplay account you will have it forever.
If you would also like Prince of Persia, Rayman Legends, Splinter Cell and The Crew for free visit the Advent Calendar for days 22-25 (December 15-18). You must claim these games before December 23, 2016 at 23:59 pm CET.
Disclaimer: This is only the base game so its DLC will not be included. If you want advice about DLC feel free to ask.
How to download?
Download and install Uplay. Sign in with your Uplay account. It will be there in the Games tab under My Games. Total size will be around 10 GB.