Apple will introduce new versions of iOS and OS X at its annual developer's conference.
Moscone Center Schedule Points Towards June 8-12 for 2015 Worldwide Developers Conference
While the aforementioned dates are earmarked as just a "Corporate Event" on Moscone's website, in previous years (2008, 2009, 2010, 2012), the same non-specific Corporate Event during the June time period has turned out to be a placeholder for WWDC.
The June 8 to 12 time period reserved at Moscone West also coincides with a multi-year convention calendar for the San Francisco Convention and Visitors Bureau that was discovered in 2010. Thus far, the listed dates, which cover potential WWDC events from 2010 to 2015, have been accurate for the past four conferences.
It is, of course, possible that the generic Corporate Event label pertains to another event unrelated to WWDC, but based on past history, it seems to point towards Apple's events. It is also possible that Apple could opt to its schedule, but at this point in time, June 8, 2015 appears to be the tentative date for the next Worldwide Developers Conference.
An official confirmation of the dates for the 26th Worldwide Developers Conference will not come until the first few months of 2015, when Apple makes an announcement and begins ticket sales. In 2014, Apple announced its June 2 WWDC on April 3. Tickets to Apple's 2014 conference were distributed via a lottery system due to high demand, and it's possible tickets for the 2015 WWDC will be handled in the same way.
Though it's early to predict possible product introductions at next year's conference, based on past history, Apple will likely highlight both iOS 9 and OS X 10.11, Yosemite's successor. The timing may also coincide with the launch of some of Intel's Broadwell chips, making it an ideal event to unveil updated Broadwell-based MacBooks.
Update 8/7 8PM PT: The "Corporate Event" listed on the Moscone Center event schedule has now been removed.