how do the camelot knights celebrate Christmas and New Year?

They had a huge spread of food, kind of like a fancy medieval buffet! It is opulent and festive. It was fifteen days of mirth (good cheer), dancing and eating/drinking,

"For there the feast held full fifteen days alike with all the meat and mirth that men could devise. Such a merry tumult..."

This would have been inKing Arthur's castle at Camelot.