I would expect the week before Easter Sunday to be the strongest trader of the year. But, whether you want to reserve a 1-bedroom holiday week or a 2-bedroom non-holiday week will depend a lot on what you want to receive in exchange. II makes it very difficult to trade up in size, so if you'll need a 2-bedroom, you should book a 2-bedroom.
If you are considering depositing a non-holiday week, I would call II and ask which weeks qualify for Accommodation Certificates (bonus weeks.)
You can also check the II demand index for Mexico. This has some value, although I don't find the demand index to be completely reliable.
You may also want to ask again here next spring (for your 2011 week.) The strongest week for your Mexico resort will
certainly fall sometime between Christmas and the week after Easter. Ordinarily, an II member could just look at that time frame, and tell you what dates have availability in PV. (The dates with II availability would be the dates to avoid.) However, since we are already within that period, much of the inventory for the upcoming season will have been booked, even if it's not particularly high demand,and not much for 2010 has been deposited. Checking further out (say, next May to August) will provide a better idea of what the high demand times in PV are.
....Personally I prefer a week that starts on a Saturday, but I don't know if it makes a difference between Fr/Sa/Su, perhaps others will chime in on that one.....
I've heard that Saturday is highest, followed by Sunday, with Friday a distant third.