I tend to plant mine in mid to late September. I try to time it so I have the plots worked up by the second weekend in Sept, and then plant when there is a pretty sure chance of rain within a few days. Of course, it doesn't always work out like that, but that is my goal.
Sometimes you can get "old" seed (seed they might have left over from the previous year) from the coop or county feed store for less than half price, and I find that germination is almost as good as "fresh" seed. I do overseed by about 25%, so that cuts into the savings, but you still end up saving a little bit. Last year I planted a mix of purple top turnips, mustard greens, DER, and oats in a one acre plot, and the deer seemed to like the variety of greens. The turnip and mustard green seed was from the previous year and it still made a nice thick plot (until the deer decimated it in December).