early...I like it in the -5 to -15 for action as well as comfort. It's true as Skeeter says youcan see them anytime, but I have found that when the temp's drop they are really travelling alot more during all hours of the day. When it's warm the action usually seems to drop drastically. Also, too cold just makes for downright bad tree stand hunting, because no matter what you wear, you aren't moving so the chills set in after a few hours and that's not very fun. Blinds are way better in really cold conditions for me. But I definately agree...fresh snow is your best hunting partner! It makes them move, shows you recent travel patterns and is great for tracking a wounded animal.
JDinAB