A recent tweet from “Paul Thurrott” puts Windows 8.1 update 1 release to general public on 8th April. Developers will however be able to get their hands on the update one week before launch, as the update is hitting MSDN on April 2nd.
Windows 8.1 Update 1 will hit MSDN on April 2, Windows Update on April 8
— Paul Thurrott (@thurrott) February 27, 2014
Other sources like Mary Jo Foley from ZDNet have also mentioned a target release date of April for Windows Phone 8.1 update 1.
At MWC 2014 , Joe Belfiore from Microsoft has talked about the changes that this update is supposed to bring. Lots of those changes go in enabling low-cost hardware and UI improvements targeted at making life easier for mouse and keyboard users. You may like to see the update in action in the demo video by clicking here. Joe Belfiore has also mentioned release date of Spring for the update.