I was going to ask the same question, but since you already did, I'm just going to post my insight/guess on this subject.
With auto, late December is the best time to buy. It's because dealers are trying to hit a specific sales number for the year so they can get a bigger bonus payment from the Manufacturer. Out side of that, end of months and fall is usually best. End of month because dealers want to clear out vehicles before they have to pay possible floor plan (Some manufacturer's also run monthly bonus problems too) and Fall because they want to clear current model year for the next years models that are coming in.
Boats are pretty much them same, but for slightly different reasons. From what I found with boats is that the best time is late winter because of incentives, but floor plan and other things apply.
Being new to tractors, I have no idea. I'm guessing incentives happen quarterly. Sometimes they get better, sometimes worse. It probably depends on some number cruncher that's sitting behind a desk looking at how many they want to sell versus what they expect to sell. With the way the economy is going, they might come out with a 60 months at 0% like Montana Tractors currently has.
BTW if there's no better time to buy, then it works the other way to, as in why by now if nothings going to change??? But it's probably going to depend on where in the country one lives. Like others pointed out, in the north, you use the tractor for plowing snow, but outside the snow belt, if it's to cold to farm or work outside, whatcha need a tractor for??
