This block off of vacation dates will be for September, which is presumably when Apple will be launching their next-gen iPhone. September seems to be about right, since Apple has typically released their phones around then. At the same time we have heard rumors that the 4.7-inch model could be launched in August, but perhaps its release will be in September, thus lining up with the recent claims.
This is a similar move done by Apple back in 2013 where they blocked off vacation dates from the 15th of September until the 28th of September in conjunction with the release of the iPhone 5s, iPhone 5c, and iOS 7. In any case it might be a bit too early at the moment to determine if the rumors are true, but do take it with a grain of salt for now, but you can rest assured knowing that we will be keeping our eyes peeled for additional information.