Early last week, I got a hold of Universal by phone and they said that YES - you can buy a ticket for the last day (December 17) and that is valid for 7 days (from December 17 - 23).
You do NOT have to go the first day - you can go ANY 4 days. You could go December 20, 21, 22, 23 if you wanted to.
The 'actual' block-out dates are December 24-December 31, which is why they stopped the first day of tickets for the deal to 8 days before December 24.