User blog comment:CNBA3/Gray Spin Off series!/@comment-1359322-20140428090522/@comment-7418318-20140428100152

The core of the magazine will be a story about Mavis called "Fairy Tail Zero" and as the anon already said, it will also contain concept arts, some extra stuff and stories drawn by other Mangaka and edited by Hiro Mashima.

If I remember Mashima said it will come out monthly, but no releasedate for the first issue was given yet.