It's technically "late May." So it's not too surprising that Microsoft's promised late May rollout of the Windows 10 May 2019 Update (also known as 1903) is kicking off today, May 21.
As of today, mainstream consumer and business users who want to manually download and install the May feature update may do so. The May 2019 Update/1903 is available on WSUS, Windows Update for Business as of today. Users who aren't already on Windows 10 1809 (either because they weren't "offered" it or didn't proactively grab it) will be able to just skip over 1809 and go straight to 1903, since Windows 10 feature updates are cumulative.
Also: Windows 10 version 1903: Is it safe to install yet?
There are a quite a number of new features in the May 2019 Update/1903. Microsoft is providing users -- including Home users -- with more control over how and when Windows 10 feature updates will install with this release.
Microsoft is adding the ability for Home users to pause updates for up to 35 days. And thanks to a new "download and install now" option, users won't automatically be pushed to do a feature update just by the virtue of "seeking." Starting with the May Update/1903 Update, Microsoft has added a new "download and install now" option to Windows 10 1803 and 1809 as well. If a user never selects the "download and install now" option, s/he will be able to delay deploying a Windows 10 feature update almost right up to the time that support for it expires. (We don't know exactly how many days before the support period ends that users will be pushed to upgrade.)
This feature update also includes a new "reserved storage" feature that is meant to improve the success rate of installation of feature updates on machines with limited storage by allocating about 7 GB of storage specifically for updates. The May Update includes a feature called Windows Sandbox, which allows Pro, Enterprise and Education users to run potentially risky software in isolation. The May Update adds a new light desktop theme option, the ability to uninstall more built-in apps and a simpler default Start menu on new accounts/PCs. It also separates Cortana from Search and introduces the new unified Search box which Microsoft has been touting for a while now.
Microsoft officials said in early April that the company was changing the way it would roll out its May/1903 feature update in the name of improving quality (and to avoid repeating the troubled roll-out process which plagued its October 2018/1809 feature update). Instead of following the usual script -- pushing out a new Windows 10 feature update a few weeks after it was designated "done" (which happened in March, 2019, this time around) -- Microsoft would be taking a more cautious approach by testing the May update for a longer period of time within the Release Preview Ring, inside Microsoft and among its OEM and ISV partners. In a move that surprised a few of us (yours truly included), Microsoft began delivering 1903 to MSDN subscribers in early May.
Also: Windows 10 May 2019 Update: Everything you need to know TechRepublic
Officials said last month that business customers would be able to start their targeted deployments in late May. That date is today, May 21. This is the date when Microsoft will start the countdown of the 18-month servicing period for Windows 10 1903 in the "Semi-Annual" channel.
Microsoft still hasn't disclosed how and when Insider testers will get 19H2/Fall 2019 test builds. Usually, Insiders already would have gotten their hands on these bits by now, given the May update "RTM'd" back in March. But so far, Microsoft has been testing its 20H1 update, not 19H2. My contacts say this is because the 19H2/Fall Update for Windows 10 will be fairly minor and will include some features back-ported from 20H1 (which may be turned off by default).
Hopefully, we'll hear more (officially) about the 19H2 testing plans "soon" (tm).
Windows 10 May 2019 Update: The new features that matter most