A reservation needs to be cancelled AT LEAST 31 days before check-in date for the
CURRENT USE YEAR POINTS to still be bankable & it must also be the first 8th months of that UY for those current UY points to still be bankable. If you also utilized points banked from previous UY &/or borrowed from upcoming UY for reservation those points must be utlized during the UY the reservation is in regardless of when you cancel because banked or borrowed points can never be "put back" into the UY that they were initially from.
You may always make a reservation at your Home Resort 11 months before check-in date you want and as early as 7 months before check-in date at non-Home Resort you wish to stay at. Points to cover your reservation will be from current Use Year allotment &/or from points you've banked from previous UY or that MS will borrow from your upcoming UY.
Since there isn't a May Use Year and you plan to mostly travel in June imho then June is your best pick for Use Year - you will get your allotment of points on June 1 each year so for example
~ on June 1 2009 receive 2009 UY allotment of points
which you will utilize for trip on June 21 2009
but you need to cancel
and if you do so on May 21 2009 or earlier (which is 31 days or more before check-in date & allows these points to still be bankable to 2010 UY )
~ THEN you would either have until May 31 2010 to utilize these
2009 Use Year points OR you'd have until Jan 31 2010 to BANK
these points into June 2010 Use Year.
But if you cancelled between 30 days to 1 day before check-in although you wouldn't be able to bank these points you would still have until May 31 2010 to utilize the points which is the last date of your current 2009 Use Year.
Also if this reservation was going to make use of points you ha banked from June 2008 UY &/or borrowed from June 2010 UY then these banked or borrowed points would need to be utilized no matter when cancellation was made no later than May 31 2010 which is the last day of 2009 Use Year since these points cannot be banked or put back into the Use Year they were initially from.
