While the weather is still nice, check your horse's winter housing. Clean out the run-in shed. Check drainage to be sure water won't run into the shed. If more than one horse will be using the run-in shed, allow 80 square feet per horse. Make sure there are no loose boards or nails in the shed.
Horses are active and prone to injuries.
If you plan to put your horse in a stall when the weather gets cold, let it go into the stall now. Also, clean out the stall, and check it for needed repairs and remove risks. After the housing is in shape, check your horse. It should have enough body fat to insulate against winter's cold wind, rain and snow. You can visually evaluate body condition.