spikes the eggnog? Seneca, of course
hangs the stockings? Katniss (after being badgered by Seneca)
leaves cookies and milk out to keep up with the santa tradition? Seneca (Katniss thinks it's a waste)
"accidentally" throws away the fruitcake: Katniss because Seneca doesn't want to touch it and it's starting to gather flies.
goes overboard on the whipped cream? They both do (only because Seneca sprayed it in Katniss' face and she threw it right back at him)
is the inevitable Christmas grinch? Katniss; she just misses her family
wakes up first on Christmas? Surprisingly, Katniss.
rolls their eyes as the same cheesy carol plays in the stores for the millionth time? Katniss; she doesn't understand how Capitol people could like the poppy Christmas tunes.
starts playing Christmas music the day after Halloween? Seneca (if only to tease Katniss)
decorates the Christmas tree? They both do with both Capitol ornaments and traditional District 12 decorations.