USe year has nothing to do with when you can make the reservation. As you surmised, you do not need to have points in your account at the time you call to make the reservation - the points must be in the account on the day you arrive.
If your use year is January, your 2003 use year would run from 1/1/2003 through 12/31/2003.
If your use year was April, your 2003 use year would run from 4/1/2003 through 3/30/2004.
If you wanted to stay 7 nights beginning 12/13/2003, the first day you could call to make the reservation (assuming you only wanted to make one call) would be January 20, 2003. That's because 1/20/2003 is 11 months ahead of your desired check out day. It doesn't matter if your use year is January or April (or any other month). The December stay in the example falls within the 2003 use year. To pay for that stay, you can use either 2003 use year points, points that you banked into your 2003 use year from your 2002 use year, or points that you borrowed from your 2004 use year.
To sum it up, you can always make a ressie at your home resort 11 months ahead of check out as long as there will be points in your account when you arrive.
HTH.