Where are you seeing that the 28th is a blackout date?? The 25th is as Tumbleweed said because it's a heavy travel day and Amtrak wants people to pay for fares.
People want to be where they're going before the holiday and leave after the holiday. That is why you see blackout days before the holiday and after the holiday, but not on the actual holiday!
Using Thanksgiving as an example, those coming in want to spend the day with the family, not leave on a 8 AM or 10 AM train on Thanksgiving morning. They may want to spend a few days with the family. So Amtrak blacks out the days before and after Thanksgiving for AGR awards, but not Thanksgiving day. Amtrak would rather get paid for travel on those days. (And can you blame them?