Agree with Sept. In general you want all your travel to occur early in your UY, but you want to avoid reservations in the 3-4 months before your UY starts (ends). When it comes to cancelling reservations there are 3 things to consider:
A. For all points cancelling in the last 30 days puts the points in holding - means you can just book villas 60 days from the time of booking. These reservations can be tough to find, so if you end up with points in holding close to end of your UY, they are at risk of being lost.
B. The banking window is 4 months before the end of the UY. If you had a reservation in the last month of your UY, and you cancelled it just before the holding window (31 days early), you still would be hard pressed to use the points. For Sept UY you need to bank by 4/30, so you would need to cancel by 4/30 to be able to bank current UY points. So if you had a 5/29 reservation you cancelled by 4/28... The points would not go into holding and you could bank the points.
C. You can only bank once and you can't unborrow points. So, even if you cancelled that May reservation in April... For current UY points - you are fine, they can still be banked, however if you were using banked or borrowed points for that reservation, you still have to use those by 8/31 when they expire. That's why it is best to book early in the window. Say you still had Sept UY and you had a trip planned for Sept/Oct using banked and/or borrowed points. If you cancelled that trip before the 30 day holding window in July/August, you have all the way till the next 8/31 to use the points. You can still take advantage of the 7-11 month booking window and plan your trip replacement for the next April-August.
Since you travel from in Jan, March, May, Sept, Oct and Dec.. the only 4 month gap is May to Sept....