As you know, you need to buy your fall seeds ASAP. It may be even too late, depending where you are and what you want to plant. I've been having a hard time figuring out when to actually plant fall crops. The problem is that after a frost, some plants will die. So there is a very final part of the growing season.
For Arlington, it looks like the average first frost date is mid-October.
Um, ok. So what does that mean? Different plants have different germination times and different number of days to maturity. It can get very confusing, very fast.
I was scratching my head about this whole thing & decided to procrastinate by looking at gardening blogs. What I tend to do is find a blog I like, then read all their posts. THEN, I look at their blog roll, find a blog from there I like, and read all those posts. This method, unbeknown to me, lead to my solution. I stumbled across Skippy's Vegetable Garden blog....which had a Fall Planting Calendar.
Just put in your average last frost date & click "Generate Fall Planting Calender"! Score!
I'm a little late on some things, but I wasn't planning to plant them anyway. I love how this takes a large hunk of the guesswork (and math) out of fall crops.