The first Steam Next Fest (opens in new tab) happened this June and among the 700-odd (!) demos available, there were more than a few gems to discover (opens in new tab). Valve announced shortly afterward that it would host another next month, running from October 1-7, and now the event has a trailer showing off some of the games being featured.
Steam Next Fest is, as the name suggests, all about what's coming down the line: the idea is that a whole bunch of demos for all sorts of games land at the same time, and then you can fill your boots for the duration. Or if you're sensible, wait a few days until we've sifted through 'em all and can tell you what to look out for (opens in new tab).
The event doesn't have a schedule yet but will feature developer livestreams and interviews about games in progress. There's no firm number of games yet beyond "hundreds", though expect more than you can reasonably play in a week. That's not to be down on Next Fest though: for as long as Steam has been Steam smaller developers have had issues getting their games surfaced in front of people, and regular events like this must help with visibility.
Valve will begin the event on October 1 with its own livestream, at which it will no doubt announce Half-Life 3, and you can set a Steam reminder for the event here (opens in new tab).