„An diesem Tag, dem 31. Dezember eines Jahres, wird um Mitternacht das neue Jahr begrüßt. Die Deutschen tun dies traditionell (*) mit Feuerwerkskörpern, die wenige Tage vor Silvester in Spezialgeschäften und Supermärkten erhältlich sind.“
„Dies“ could be replaced by „das“ and it refers to the welcoming of the New Year. You could also say: “Die Deutschen begrüßen traditionell das Neue Jahr mit Feuerwerkskörpern …”.
„Traditionell“ means traditional and has to do with history. It says that people have done something for quite a long time so it has become part of their life. “Traditionell” implies a kind of behaviour that has been carried out for quite a long time, perhaps for some generations.
Regards,
Paul