I would have to agree with CarolMN's assessment. I would definitely do a day by day call for the week between Christmas and New Year's.
Connears, while agree in part with your statement, what would happen if you didn't get the room you were looking for? You'd probably be sitting their kicking yourself for not calling day by day.
Yes...there are times when calling day by day serves no purpose, but when I made my reservations for early December, I called every morning at opening time. Was always greeted cordially by MS, and I didn't get the impression that they were overwhelmed by me calling every day. In fact, when I called on the first day they started off by asking me if I was adding on to an existing reservation or starting a new one.
I have a feeling MS knows when DVCers will be calling day by day @ 11 month windows. I did get busy signals a couple of times, but waited 5 minutes and got right through.
I would seriously doubt that you'd have to call day by day for a room after Easter, but if your trip runs into any portion of the Easter week, I'd consider it.
Brian