Use year does not effect when you can go or when you can call to reserve. Its potential impact has to do with banking points if you have to cancel. You can bank all your points from one year to the next at anytime during the first 8 months of use year but not thereafter. Impact on banking by cancellation:
1. Assume Sep use year and Aug trip (at end of use year). In June you realize you must cancel. Points come back into account but because you are beyond the 8th month of your use year you cannot bank and they must be used by end of Aug.
2. Now assume Sep use year and December trip (fourth month of use year) If you have to cancel in October, you still have plenty of time to either use the points for another trip in the same use year or bank them into the next use year.
3. Thus, if you usually travel anytime in summer, including Sep, safe use years to maintain the cancellation/banking hedge are Feb to June with June ideal and Feb within the safety range.
You can request different use year than one they are selling at time you plan to buy and, if it is available, they will sell it to you.