The translation su agrees in number with the noun which it modifies; their is translated by su, sus, depending on what follows: their father/mother su padre/madre; their books/magazines sus libros/revistas. For their used with parts of the body,
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Sanfermines (The festival of San Fermín) is from 6th-14th July and el encierro (the 'running of the bulls'), takes place in Pamplona in northern Spain. The animals are released into the barricaded streets and people run in front of them, in honor of the town´s patron saint, San Fermín, who was put to death by being dragged by bulls.