The best advice is book something in advance if you can. When you're in Amtrak's reservations system the web, make sure you're not booking a train with a four digit number (excepting the Acela's, of course). The holiday extras are usually in the 11xx series or the 30xx series, with I believe the 30xx's most often being commuter equipment. Last year's holiday schedule seemed to indicate that the 11xx trains were second sections of Regionals that often operated the full DC-BOS.
I did a quick check of searching for a reservation from Newark to DC on Thursday and Sunday, and also Friday. The system pulled up a couple 30xx's on Thursday and Sunday, and did not identify them as "Regional Service" but did indicate a food service car. Might be the return of the "enhanced MARC train" (MARC set with a Amtrak cafe) as seen before. Friday, however, showed nothing but Regionals, Metroliners, and AE's.
Perhaps we should expand this to include other Thanksgiving travel tips for the NEC and other parts of the system? All I can say is that this is the weekend to treat youself to an Acela if you can afford it. I will be going BOS-BWI again this year, repeating my trip on Wednesday's #2153 SB and Sunday's #2252 NB. I'm keeping my fingers crossed for as smooth a trip as last year.