When you travel over two UY's, it can be a little bit more tricky but the easiest way to remember it for me, is to first figure out the UY the days of travel fall and go from there.
In your case, as mentioned, March 31st takes place in your 2011 UY--therefore you can use banked 2010, 2011 or borrowed 2012 points for that day.
April 1st through the 6th take place in your 2012 UY--therefore, you can use banked 2011, 2012 or borrowed 2013 points for those days.
If your plan is to use 2011 points, then you will have to bank all but the amount needed for March 31st, and then book.
Good luck!