Last week, Microsoft finally announced the Windows 10 April 2018 Update, and said that it will be available on Monday, which is today. The company promised a similar delivery method to what happened with the Creators Update, or version 1703. It said that you'll be able to manually install it on your machine beginning today, and it will roll out broadly via Windows Update beginning on May 8.
That's not exactly what happened, as you can grab Windows 10 version 1803 right now via Windows Update. This should still be a staged rollout, but if you go to Settings -> Update & security -> Windows Update -> Check for updates, you'll likely find the update waiting for you.
In a blog post about how to get the April 2018 Update, Microsoft clarified that while you can manually check for updates to get it, it's still going to be rolling out globally on May 8. This means that if you're not the type to go and manually check for updates, that's when you should start to see it automatically.
If you don't see the update yet and you want to install it by other means like the Update Assistant or you want to do a clean install with the Media Creation Tool, check out our guide here.