According to the Appvent Calendar features along with daily free game of the day, UniWars, Monster Ball, Trigger Happy Christmas, Minim, and AlphaBattle are all free. All games look to be very much worth a download, and I can pretty much assure you that both Trigger Happy and UniWars are both worth a download.
If you don’t know what the Appvent Calendar is, it introduces a free game everyday that will stay free for only one day. UniWars has been specified as free for 24 hours only, but featured free games on the Appvent usually last longer than the given one day only. There will be another free game revealed tomorrow, but for today, many games have gone free.
Enjoy, and have a merry Christmas!