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.
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'; in Galician 'Bo Nadal'; and in Basque (or Euskara in basque) 'Eguberri on".
December 28th is 'Día de los santos inocentes' or 'Day of the Innocent Saints' and is very like April 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 with the 12 strokes of the clock at Midnight! Each grape represents a month of the coming year, so if you eat the twelve grapes, you are said to be lucky in the new year.
Apart from Christmas, there is another festival that is celebrated in Spain that is about the Christmas Story. It is called Epiphany and is celebrated on 6th January. In Spanish, Epiphany is called 'Fiesta de Los tres Reyes Mages': in English this means 'The festival of the three Magic Kings'. Epiphany celebrates when the Kings brought gifts to the newborn.
Children have some presents on Christmas Day, but most are opened at Epiphany. Some children believe that the Kings bring presents to them at Epiphany. They write letters to the Kings on Boxing Day December 26th, asking for toys and presents. And on Epiphany Eve (January 5th) they leave shoes on windowsills or balconies or under the Christmas tree to be filled with presents. Gifts are often left by children for the Kings, a glass of Cognac for each King, a satsuma and some walnuts. Sometimes a bucket of water is left for the camels that bring the Kings! If the children have been bad, the Kings might leave pieces of coal made out of sugar in the presents!