Yikes. That's a tough call as it's generally peak season for the Asia flights especially with all the foreign students headed back to Asia for the holidays. I don't know how flexible you are, but there are 2 seats on SQ 631 NRT-BKK on 12/20. Don't see anything else on 12/19 other than Y.

Just to clarify there are at least 2 seats on SQ 631 NRT-BKK in Business on 12/20.