The Fedora 21 release was delayed by yet another week due to unresolved blocker bugs. In particular, there's 8 blocker bugs right now ranging from Anaconda installer issues to theming issues. As a result, Fedora 21 Alpha will hopefully come around the 16th of September if no further delays take place. All further milestones are pushed back by an additional week.
This delay means that Fedora 21 could now be further complicated by the US Thanksgiving holiday and any further delays will push the F21 release into December, compared to the original plan of releasing by late October... More delays are very well possible given that there's already been several F21 delays without getting out an alpha release while also needing to go through the beta and final release processes.
Confirmation of this latest Fedora 21 delay can be found via this mailing list post.