The Xbox's titles have gone public and it's a pretty extensive list, especially if you are still rocking a 360.
First off, the fairy tale RPG Child of Light and Pool Nation FX are your titles for Xbox One through the month. While I didn't love all of Child of Light's mechanics, there is plenty to enjoy here. I'd certainly encourage you to give it a try.
If you are on Xbox 360 mind, you've got four titles headed your way. Gears of War: Judgement and Terraria will be available 1st-15th of April. After that, Assassin's Creed: Black Flag and Army of Two: The Devil's Cartel will see out the month.
That is a lot of game for free. The highlight is easily Black Flag though, which may be the best game in the Assassin's Creed franchise. Make sure you pick that one up.