https://www.pcgamer.com/arma-3-is-free-to-play-for-the-weekend/

If you're the sort of shooter fan who looks upon a game like Day of Infamy as too casual to bother with even when it's free, you'll be happy to know that Arma 3, Bohemia Interactive's realistic modern combat FPS, is also free to play this weekend on Steam. 

The freebie actually started a few hours ago, and will run until 10 am PT/1 pm ET on April 30. It includes the full base game, with all "vanilla" weapons and vehicles, plus the Scenario Editor, all multiplayer modes, and the Bootcamp Prologue and The East Wind single-player campaigns.