When you bank points they are effectively moved from one Use Year to the next.
Points from your 2005 Use Year (5/1/05 to 4/30/06) are moved into the 2006 Use Year (5/1/06 to 4/30/07.)
Edit: I don't believe there is a May Use Year, but since your question was theoretical, you should get the point. Also, you have the first 6 months of your Use Year to bank 100% of your points. Keeping with your example, for a May UY you would have until October 31 to bank all of the points.