As to what use year month you get, when you buy from Disney it is usually one that starts 4 to 5 months before the date you agree to purchase. As a result, when you buy you get a full allotment of points and then 7 to 8 months later you get your second allotment.
Use year month has nothing to do with when you can go (which is anytime of year) or when you can call to make ressies (always 11 months in advance at home resort 7 at others). What it does determine is when you get your annual allotment of points (the first day of the use year) and by when you need to bank points if you intend to bank them from one use year to another. If you actually do usually go only one time of the year and intend to follow that pattern for a long time, then you should consider making your use year month 5 to 6 months or less before you usually go. For example if you usually go in May, you should consider a month from December or after but not past May. The only reason for that is a hedge against the risk of having to cancel a trip. You have until the 6th month of your use year to bank all points into the next use year. If, for example, you have a Feb use year, trip scheduled in May using points issued Feb, and cancel end of March, you will still have 10 months to use those points or 4 months to bank them into next use year. If instead you have a June use year, make a ressie for May using points issued the prior June, and cancel in March, you are faced with having to use points by end of May and you are beyond that 6 month banking date and cannot bank all your points.
Note, the cancellation issue is essentially the only resaon to have any care whatsoever about when your use year month is and if you actually go different times of year then it is not even worth the bother thinking about that issue.