When is the cheapest time to fly in September?

Wednesday.

Most business users will fly on a Monday or Friday (there will, of course, be exceptions to this).
Most commerical users will fly on Saturdays (working 'til Friday) or on Monday (start of paid Holiday from work). Don't forget to consider what the Schools/Colleges/Universities are doing when you book since travel increases during holiday periods (Christmas, Thanksgiving, Easter etc.) and drops off during Term time.

I hope this was helpful,


Professor Raddish