The only text in the teaser reads "Upgrade your experience" and "May 2019," so presumably that's when the Index will go on sale, given that the word "sale" is in the URL.
(A point of clarification: The HTC Vive is commonly considered 'Valve's headset' because it's the primary SteamVR headset, but it's made by HTC. Any company can produce SteamVR-compatible gear, and the idea was that multiple manufacturers would create a variety of headsets for the platform. That hasn't yet manifested, with Valve's own Index being the first big Vive competitor to appear. Valve is also working on some new SteamVR controllers, and other companies can also do that.)
Valve's headset has been rumored for a while, and became just about a sure thing when convincing-looking images leaked last November. Before that, in March of last year, Gabe Newell said he was jealous of Nintendo's ability to produce both hardware and games, which was a strong hint. Valve also announced in 2017 that it's developing three new VR games, and presumably one or more will be tied to the Index's release.
Additionally, SteamDB took note of the Steam database listing earlier this year, and found the logo in a trademark filing before we knew what it referred to:
Even though it was heavily rumored, I'm a little surprised we're seeing Valve's headset so soon. Over the past several years, Valve seemed to be flirting with the idea of becoming a hardware company, but not fully committing. With its own headset, it's taking a big step toward those Nintendo dreams. (Also, the "upgrade your experience" line is bold. I like it. Wonder how HTC feels, though?)
And yeah, we are aware that it's almost the First of April, but Valve tells us "it's real" and that there'll be "more news soon." (Also it'd be a bit weird to announce a VR headset that looks just like those leaked photos as a joke.)