You do not need reservations on all trains between Brussels and Amsterdam but it is recommended. Some trains however DO REQUIRE reservations like the 11:25 AM train. I usually take the 10:46 AM which does not require reservation. If you are worried about seating, most of the trains originate at Brussels Midi then stop at BXL Central and BXL North. I take the train at Brussels Midi and threfore have plenty of seats to choose from.
Don't worry about the 26, you are right in the middle of the holiday ! People will take off evening of 24 and come back on Sunday