Apple doesn't pre-announced their World Wide Developers Conference (WWDC), but they do have to schedule the traditional Moscone West venue and any and all other logistics involved in hosting 6000 of its closest developer and designer friends. That leaves a week-long gap in Moscone's schedule, and that leads to people speculating any such gap in June might be for WWDC. Such is the case for the week of June 8, 2015. Macrumors did the sleuthing:
Apple's 26th annual Worldwide Developers Conference will most likely commence on June 8, 2015, and last through June 12, 2015, according to dates blocked out on the Moscone Center's 2015 summer schedule.
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.
WWDC 2015 would likely include the first public looks at iOS 9, OS X 10.11, and more.
Of course, we'll likely not get any official word on the dates until April 2015, and while past behavior is often the best indicator of future behavior, Apple can and sometimes does change things up.
So, if you're tempted to make plans, keep 'em flexible.