Welcome to Windows Phone Central! The current rumored date is the 14th, but please take it with a grain of salt as there's been no official date yet. Hope you enjoy your stay, and for any further questions, use the search feature and take a look around!
As Zulfigar said, the most credible date as of now is the 14th of this month. Rest assured, its certain that the preview for developers will be out in the "first part of April".
I think he was refering to the desktop update which should be available today depending on the region tomorrow....
EDIT: THOUGHT IT WAS FOR THE DESKTOP, WRONG SUB FORUM ...cant delete comment on phone....