Most families eat their main Christmas meal on Christmas Eve before the service. The traditional Spanish Christmas dinner is 'Pavo Trufado de Navidad' which is Turkey stuffed with truffles (the mushrooms, not the chocolate ones!) In Galicia (a region in north-west Spain, surrounded by water) the most popular meal for Christmas Eve and for Christmas Day is seafood. This can all kinds of different seafood, from shellfish and mollusks, to lobster and small edible crabs.
After the midnight service, people walk through the streets carrying torches, playing guitars and beating on tambourines and drums. One Spanish saying is 'Esta noche es Noche-Buena, Y no Es noche de dormir' which means 'Tonight is the good night and it is not meant for sleeping!'
A few different languages are spoken in different regions in Spain. In Spanish Happy/Merry Christmas is 'Feliz Navidad'; in Catalan it's 'Bon Nadal'; and in Galician 'Bo Nadal'. Happy/Merry Christmas in lots more languages.
December 28th is 'Día de los santos inocentes' or 'Day of the Innocent Saints' and is very like Aprils Fools Day in the UK and USA. People try to trick each other into believing silly stories and jokes. Newspapers and TV stations also run silly stories. If you trick someone, you can call them 'Inocente, inocente' which means 'innocent, innocent'. 28th December is when people all over the world remember the babies that were killed on the orders of King Herod when he was trying to kill the baby Jesus.
New Year's Eve is called 'Nochevieja' or 'The Old Night' in Spain and one special tradition is that you eat 12 grapes