During the second day of the Nintendo Treehouse Live show there was an announcement and demonstration of the demo for Monster Hunter Generations. Hype at the reveal then became somewhat muted due to the peculiarity of its roll-out, however - it first appeared in the E3 2016 Humble Bundle Digital Ticket in the $4 tier. It then became available for free to all European 3DS owners with a Nintendo Account, with Nintendo of Europe tweeting a link to the relevant game page to activate the download.
In North America, however, it's not yet available to all (it'll arrive on the eShop on 30th June), and Nintendo confirmed that select 3DS owners would get emails with two codes. It was only said on the Treehouse stream that the codes would be on the way 'soon', but thankfully it's now been confirmed that they're coming today.
As the tweet says, if you're in North America you should keep an eye on your inbox in case those demo codes finally arrive. The demo includes an offline tutorial and three challenges that can be tackled in co-op either offline or online - 'Slay the Great Maccao' (Beginner), 'Slay the Malfestio' (Intermediate), and 'Slay the Nargacuga' (Advanced).
Let us know in the comments if you've had a set of two codes, and good luck!