September : probably the most important month in the fashion industry. You can notice it everywhere : designers, fashionshows, shops, fashion magazines, ..
Vogue is one of those fashion magazines. Every month they create a new number but the one of September is every year the most important. Once, I saw a documentary about 'the making of' of the September Issue. It's really interesting to see, you'll probably find it on Google.
Here is some information : http://en.wikipedia.org/wiki/The_September_Issue