Are you coming to Milan for fun?
Milan is almost always full of trade fairs, and that fills up hotel rooms and taxis, but there's nothing like the annual fashion trade fairs (especially for female fashion) to snarl Milan's traffic and curl a few lips.
Avoid these times, if you can. How?...More......
Try looking up your planned travel dates in a couple of trade fair web sites:
M+A trade fairs, Milan page for Sept. 2012 (in English, yeah!).
Fiera Milano (official page, in English, yeah!)
Hope this helps!