During last week's general manager meetings in Boca Raton, FL, the NHL began communicating its basic framework for league expansion. This June, the league will vote on and announce its intentions to expand. If expansion is approved for 2017-18, and that is the earliest possible date at this point, teams will have a year to react and plan accordingly. An expansion draft will be held in June 2017, sometime between the end of the Stanley Cup Finals and the entry draft, and the expansion team(s) will begin play during the 2017-18 season.
SB Nation blog Pension Plan Puppets crafted an excellent primer outlining the expansion draft rules as they are known today (highly recommended read).