eat roast lamb (no mint sauce but plenty of garlic and rosemary) or suckling pig, huge quantities of seafood and some monster desserts all washed down with plenty of Cava.
The night doesn't end there ! If you feel inclined and can still walk then Midnight Mass is the traditional Spanish way of welcoming in Christmas Day.
Gifts are not traditionally given on Christmas Day but on January 6th when the Three Kings (Reyes Magos) are celebrated there's plenty more eating and drinking with time spent enjoying the holiday with family. Something the Spanish are very good at.
New Years Eve (Nochevieja) sees many Spanish Plazas full of celebrating Spaniards stuffing their mouths with grapes ! Easily something you can introduce to your New Years Eve. As the bells chime on the run up to the midnight hour you should put a grape in your mouth at each chime. It is up to you whether you hold them all there or eat them as you go - either way if you eat all twelve you can expect twelve months of prosperity in the New Year.