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You say that you know the thing about the 12 grapes. I guess, that you know too about the peals. The thing is that a lot of people go to La Puerta del Sol, (Sun“s door) before 12:00 of course, and the big clock has 12 peals that represent the 12 months of the year. You have to eat one grape with every peal, that mean, good luck for the next 12 months.
Then there are fireworks and the people drink to celebrate the new year, (I guess in all the countries do that). I do not know how to explain more, so I prefer to give here a link for youtube, that someone who was there.
Then, the people go to parties, to celebrate with the friends, until the morning.
Maybe it is a good idea, that you ask in a web of an american, who now lives in Madrid. There you can find information of first hand of americans and british who live in Madrid or the have spent many time here, about acommodation, food, practical information, etc. And the most important is that it is in english. Sometimes I read the message board and the blog. First of all it is very interesting and I love it, (he does a great work) and second I read it in english and I always learn new vocabulary.