If you buy a December UY you would get 2016 points then in December you would get 2017 UY points, but not any other UY. If you buy any other UY you will get 2017 UY Points.
This being said though, make sure you buy the best UY for your travel habits. If you travel to WDW in November, a December UY is a bad UY for you. You want a UY that begins in the first few months of your regular travel habits, because if you have to cancel a trip your points go back in to your account, and aren't put into holding or just lost.
We have a February UY because we have young children and have to go to WDW during the February and April vacation week school schedule, thus we are traveling in the first 3 months of our UY. We also bought a have a June UY which is horrible for our travel habits because it is at the end of our travel habits. If we cancel an April trip, the points go into holding and we might loss them unless we take a trip in May.
Hope this helps.