Use year is irrelevant to when you can go (any time of year) or when you can call to reserve (always beginning 11 months out at home resort and 7 at others).
Its possible relevance has to do with the combined issues of banking and cancellation. You can bank any or all of your points from one use year into the next for use for a trip in that next use year but you must do so by no later than the end of the eighth month of your use year, e.g., if your use year is June, you have until January 31 of the use year to bank into the next use year; example you have June 2012 points, if you bank them by 1/31/13 you can use them for a trip in the June 2013 use year (anytime between 6/1/13 to 5/31/14).
Now we get to the cancellation issue. Suppose your use year is June and you currently have your June 2012 points for use. They must be used for a trip that ends by no later than 5/31/13 unless you bank them. Now assume you called in late June and booked a trip using those points for the last week of May 2013. Next assume something unforseen happens and in March 2013 you learn you must cancel that trip. You do so and the points go back into your account. However, you can't bank them because March is after your last date to bank, 1/31/13. You thus have only until 5/31/2013, two months after you cancel to use them or you will lose them.
Now a different assumption using the same June use year. You call in early July for a trip to occur in early June 2013. Your "current" use year points for that trip would be the ones for June 2013. You could also bank any points you have from June 2012 and use them for that trip. Now assume you have to cancel that trip in March 2013. Points go back into your account. You would have over a year to use the banked points from 2012 and you would have until January 31, 2014 to bank the 2013 points. Result, you have no serious risk of losing those points.
As a result, many recommend that if you have a regular time of year during which you will go, you should get a use year that starts shortly before the usual time of year you go. In reality, to avoid the issue of being unable to bank if you have to cancel, you can actually get any use year that begins up to 8 months before the time you usually go but better to be somewhat closer than that, e.g., 0 to 4 months. Note, I use 0 months because if you usually go in June then June is an ideal use year because your points begin 6/1 and thus any trip in June would be using newly issued points.
Just note, however, that what is good now may not be as good later. For example, someone who goes every year in June to fit kids' schedules may in 15 years when kids are grown start going every year in late April. Moreover, the issue makes little difference if your current probability is to go at varying times of year.
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