Starting on May 2 (May 3), the Early Access feature of the Overwatch Open Beta is available to players that pre-ordered the game by April 29. Early Access starts at the same time in all regions, so, it is better to check out this time-zone map to understand when the Early Access Open Beta starts for you:
For everyone else, the Open Beta will start on May 4 (May 5) at 4:00 p.m. PDT (11:00 p.m. UTC). See this map to know the exact time of your region:
The Open Beta will conclude in all regions for all players on Monday, May 9 at 10:00 a.m. PDT (5:00 p.m. UTC)