Spring is already here, but the Spring Creators Update for Windows 10 is not, even though it was expected to release yesterday. Technically it's not delayed, because Microsoft never officially announced a release date. Unofficially however, the reason you're not seeing it yet might be because of a bug.
Citing an unnamed source, Windows Central says Microsoft discovered a "blocking bug" right at the last minute. It's not clear what exactly the bug might have been doing, but it had enough of an impact that Microsoft decided to hit the abort button right as the Spring Creators Update was getting ready to launch.
Got some more info on this: Microsoft was going to rollout on April 10, but found a blocking bug over the weekend that was bad enough to hold the release. Not sure if bug was fixed in 17133.73 or if it'll come in another patch. RS4 will likely begin rollout in a couple weeks now. https://t.co/qxcbHCdPUoApril 10, 2018
It's apparently rare to find a show-stopping bug in the 11th hour. In this case, the Spring Creators Update had undergone significant testing internally at Microsoft and by users participating in the Windows Insider program. Major releases like this one go through all kinds of tweaking and validation along the way, and by the time it's finished enough to start making its way to customers, it will have passed all kinds of stringent criteria.
What's also not clear is what effect this could have on Microsoft's OEM partners. Typically OEMs receive polished code in advance of the public so they can prep their own roll outs, update drivers, and customize things as necessary. If the supposed blocking bug is particularly troublesome, it could mean a longer delay as Microsoft works with OEMs to iron things out. On the flip side, if it's an easy fix, it could be a short delay.
The Spring Creators Update (not an official name, by the way) is the first of two major updates to Windows 10 planned for this year. Microsoft has committed to issuing two major upgrades to Windows 10 bi-annually, in between which it issues monthly security updates and other patches as needed. The Spring Creators Update will introduce several changes and improvements, including a revamped Game Bar with an integrated clock, better support for HDR displays, and more.
If you don't want to wait for the public roll out, you can join the Windows Insider program long enough to grab the latest build, and then duck out of the program once it's installed. But since we don't know what kind of bug might be preventing its release, it's probably better to just hang tight.