You can start calling at 12-months, but unless a week 44 owner specifically cancels his/her reservation, nothing will open up until the 10-month mark. In other words, if a week 44 is actually open at the 12-month mark, you can book it. But, it's more likely that it will become available at the 10-month mark (someone is likely to forget to call to hold his/her week).
You might be better off holding your week 39 ... then only release it if you can get the week 44.