First of all, I assume this is based on US timezone - not sure which one exactly, but it'd start some time around lunch on the 22nd here. The deal is also only available on uPlay, not steam or consoles.
Secondly, you have to install the demo on the 22nd - if you want to practice first, you basically have to pay for the full version ($15 USD on Steam) - or know someone that already has it.
To get the free copy, you have to complete the demo with 15 or fewer faults (which is pretty hard if it's your first time playing it), so it's not as easy as the tasks for gleam.io freebies, but it's also a far better game. Assuming you've got the bandwidth to download it, it can't hurt to give it a shot if you were interested in the game anyway.
We need a Demon's Souls free game demo challenge.