Today, Microsoft released Windows 10 build 17133 to Insiders on the Release Preview ring, which is reserved for updates that are slated to roll out to the general public. In case there's any confusion or doubt left, this does confirm that build 17133 is the RTM build for the upcoming spring feature update.
The build initially rolled out to the Fast ring on March 27, and then made its way to the Slow ring on March 30. Microsoft still hasn't made any official announcements regarding what the update will be called, or when it's coming, but now that it's in the Release Preview ring, it can be any day now.
The Windows 10 spring update, codenamed Redstone 4, contains a number of new features. The flagship feature is likely to be Timeline, which allows you to go back in time and look at previous versions of documents, as well as other things you might have been working on. But there are also various redesigns across the OS, as things like Settings, Edge, and more have been given new Fluent Design elements.
Similar to how it worked in the Slow ring, Microsoft is testing a staged rollout in Release Preview. You can manually install the build (also using our guide) by going to Windows Update and checking for updates, but it won't automatically show up for everyone in the ring until Monday.
To find out more about what's coming in the spring feature update for Windows 10, check out our guide here.