An April UY is good for vacations starting on April 1 - March 30 of any given UY. This UY gives you the most flexibility if you have to cancel a trip, as you'll have through March 30 to re-book a vacation. Or, you can bank those points into the next UY. Something to consider if you only travel once per year, though, is that if you bank points with an April UY, you'll have to travel before March 30, or you'll lose those points.
For example, if let's say you buy 250 points today and you make a reservation for this coming May (2009). The reservation you make is 210 points. Assuming you don't use the remaining 40 points, you would bank them into the next UY so you don't lose them. The banked 40 points would now be good for vacations from April 1, 2009 - March 30, 2010. Basically, you would have to travel again before (or rent the points) May 2010, or you would lose the points.
You may want to consider a Sept UY. With Sept, your points would be deposited every Sept. You would be able to travel on those points from Sept 1 through Aug 31 of any given year. You would be able to bank up to 100% of those points through April 30. If you do not bank by April 30, you would still have through Aug 31 to book a stay and use those points. This would allow you some flexibility once your kids get involved in school activities, too.
The downside of traveling in the last few months of your UY is that if you have to cancel your vacation and can't reschedule, you will lose those points. I worry much less about having to cancel my vacations than I do having a UY that dictates my travel. We have an Aug UY and like to travel in Aug. When I bank points, it forces us to travel in July because our banked points expire July 31. If I had an Oct UY, I wouldn't have the issue, because they would expire at the end of August instead.
Just as an FYI, when points are deposited does not impact when you can make reservations. At your home resort, you can book stays 11 months out and 7 months out for all other resorts.