I don't think it matters when you purchase resales, but do some research here to pick the best use year.
From what everyone has posted if you vacation every June, then buy a use year that would be say May or April. That way if you must cancel your vacation you have lots of time to reschedule.
The timing of your next trip may be the most important factor. If you want to use your points for your next trip, you will have to allocate time to find the contract you want, get the offer accepted, make it through ROFR, etc.
I don't think it matters too much. More important is to buy the use year that best suits your needs and the location you intend to stay at most. The 11 month window is very important for bookings during peak times!