In your confirmation e-mail and in the reservation terms I'm sure these surcharges were already mentioned.

Also, on the hotel front pages:
http://www.starwoodhotels.com/lemeri...ropertyID=1847
"Compulsory New Year's Gala Dinner: December 31, 2013"

On LM Angkor Wat's page you actually have to click to the Christmas page:
http://www.lemeridienangkor.com/en/festiveseason
"*** There will be a compulsory Christmas Eve dinner supplement for all guests staying over or arriving on December 24, 2013. This will be automatically added to the room bill."