If you usually travel in August, a December use year will work just fine.
If you have to cancel the vacation, you need to do it at least 31 days prior to arrival. If you do that, you can either bank the points (your deadline is July 31) or use them for another vacation that ends on or before November 30.
Use year matters because of the banking deadline. With a December use year, points used for vacations scheduled for September, October & November are more "at risk". That's because if you have to cancel, you would be past your banking deadline. The points would have to be used by the end of the use year (November 30) or they expire.
Keep in mind that if you ever cancel a vacation 30 days or less prior to arrival, the banking deadline won't matter. Short notice cancellations mean the points go into the holding account. Holding account points cannot be banked and always expire at the end of the use year. In addition, there are restrictions on using holding points to make a reservation.
If you cancel 5 days or less prior to arrival, the points are forfeited.
Also keep in mind that your travel habits may change over the course of your membership. A use year that starts out "perfect" may end up to be "not" perfect.