You'll never guess what this Fest is about.
Art showing a first-person view and text saying the Steam FPS Fest begins April 15 and ends April 22.

From April 15 through April 22, Valve will host the Steam FPS Fest. Contrary to what you thought "FPS" stood for, it actually stands for "first-person shooters." Yes, an entire week of Steam offering deals and demos for a variety of first-person shooter games.

Valve says that this Fest will cover everything from hunting games to diving games, and modern games back to prehistoric shooters. They're all included in this Fest so long as they are in first-person and probably contain some sort of "shooting" mechanic (and if the developers/publishers opt to participate).