There were very strong rumours it's today, but with no announcement yet I can't see it happening. Nintendo have been tweeting things that are obvious teasers, and GameStop tweeted (and quickly deleted) saying it's a 45 minute long presentation which is cool. It could get announced later today for tomorrow, but right now I'd expect it to happen next week. They don't normally do them on weekends, but I suppose Saturday is possible too.
Metroid Prime 4 might be pushing it a bit, along with Bayonetta 3 and Pokémon. I could be wrong, they may show something on them, but Nintendo tends to only show things coming in the near future and I highly doubt we see any of these three before November. There are plenty of games I can see information being given on though, such as:
Kirby
Yoshi
Fire Emblem
Animal Crossing
Bayonetta 1 & 2 on Switch (Wii U Ports)
I think the DLC for Xenoblade 2 will get more details
Maybe a couple of bigger first party games
I expect something like a Mario Maker port, maybe Smash Bros. I think that's all that's left from porting Wii U stuff that people would actually buy.
Some system-level stuff such as more information on the Paid Online Service and maybe Virtual Console, App Support (NetFlix, Twitch etc), and Home Screen Customization.
Third party stuff such as GTA 5, Lost Sphear, Bravely Default/Second ports (or Third, new game), Burnout, Wolfenstein 2, MegaMan Legacy, Street Fighter, maybe Dark Souls remastered. There will be a fair amount of this kind of stuff, but probably not everything I mentioned there.
I also think there will be more 3DS stuff than people expect. Pokémon Crystal on the eShop is this month, maybe that Detective Pikachu game.. There's something from Level 5 that I can't quite remember the name of that did well in Japan that could be localized now, and there's also the latest Yo-Kai Watch game that isn't out here yet.
My main prediction for this Direct is that people will be disappointed because they're too hyped up
but hopefully it's good!