Officially, if you want to book a stay starting Dec 25 for 7 nights, you would go online on Jan 25 and book it. Simple.
But...if you go online on Jan 19 and book a stay starting Dec 19 for 7 nights, you can grab Dec 25 and make sure that no one can book it ahead of you.
Once people figure out that they need to get Dec 19, maybe they jump online on Jan 13 to book Dec 13 plus 6 nights...
What creates the complaints is that these owners are booking nights all through December (in this case) which they don't intend to use. So someone looking to book Dec 16 for 3 nights might have some hassles waiting for the walkers to pass before their nights will open up...