During the warm months of summer, you can body clip your horse any time you want, but quit clipping by mid to end of September if you want him to grow a good winter coat that protects against the cold weather ahead. If you decide a winter body clip (or partial clip) is in order, a rule of thumb is to clip in October and again in early January if needed; stop clipping by the second or third week in January to allow his summer coat to grow in.
Click here for more information on clipping.