It is usually recommended that you get a use year that begins not too long before you usually go if you usually go during a particular time of year. If time of year you go is usually between Sep and Dec, then Sep is an ideal use year. At the other end, if the time you usually go is July or August, then Sep would be your last possible choice.
Regardless of use year, you can go any time of year. Moreover you can always call to reserve 11 months out for home resort and 7 for others. The reason for the above recommendation has to do with the risk of having to cancel a trip. Use year determines when your annual allotment of points become available for use, the first day of the use year, and by when you must bank points if you want to save them for use in the next use year, by end of eighth month of use year. Let's assume a Sep use year and a Sep trip. In Oct 2011, you call and reserve time in Sep 2012 using points coming Sep 1,2012, which will be your current use year points for that trip. Come June 2012 something happens and you have to cancel. Points go back into account and you still have until Aug 31, 2013 to use them and until April 30, 2013 to bank them into the Sep 2013-14 use year.
Now assume you have a Sep use year and you want to go in Aug 2012. You call in Oct 2011 and reserve and you use points from Sep 1, 2011 for that trip because those are the current use year points for that trip since the use year ends Aug 31, 2012. Come June 2012 you need to cancel. Points go back into account but you cannot bank them because you are beyond the banking deadline and you will lose them unless you can use them for a trip that ends no later than Aug 31, 2012.