Most people in Spain go to Midnight Mass or 'La Misa Del Gallo' (The Mass of the Rooster). It is called this because a rooster is supposed to have crowed the night that Jesus was born.
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 be all kinds of different seafood, from shellfish and mollusks, to lobster and small edible crabs.