I am nowhere near the great experise you will find here but this is what I know.
It starts when you arrive and is good for a certain number of days. I think 7 or 10. As long as you check in by Oct 2nd, it starts the night you check in but no later than than the date set by Disney.
So yes, if you check in Sept you are covered under FDP. I sure wish I knew the dates better but it is on the website.
Angel's momma is right. You must check in by 9/30 and can have a stay of a maximum of 10 nights. Your entire stay is on the free dining provided you check in by 9/30 with an applicable package.
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