It's not just the MLK holiday. Apparently the calendar for the entire year wasn't yet loaded. For kicks I tried a reservation for July 4th, a Friday in 2014, and it shows a regular weekday choice of trains. As long as one has the capability to make reservations 11 months in advance, then the system should "know" which days are designated as holidays as soon as those dates become available for advance booking.
This has become an annual routine for me for Thanksgiving or Christmas since I moved to Philly, as I must work on Black Friday (or Boxing Day when Christmas falls on a day like Wednesday). I reserve a weekday train to return to Philly on Thanksgiving night and then in the middle of October I get a robocall saying my reservation has been changed to the closest holiday schedule train with just a few minutes schedule difference. I keep the advance low-bucket fare even though the buckets change quite a bit when they program the holiday schedule.