For people updating the usual way, the target date is 4/11/17.
If you can't wait until then, Then you'll be able to download it on 4/5/17 (per Neowin):
"The method will be the same as it was with the Anniversary Update; you'll be using the Update Assistant application, which you'll be able to download here. The only steps necessary are to install it and run it, and the Creators Update will begin downloading." - Neowin
Those participating in the Windows Insider Program can download it now.