From Wiki
The traditional suit worn by women on Maundy Thursday (and sometimes on Good Friday) is known as La Mantilla (the mantle). This custom has become revitalised since the 1980s. The outfit consists of the lace mantle, stiffened by shell or another material, and a black dress, usually mid-leg, with black shoes. It is expected for the woman to hold and show a rosary. Jewellery may include, at most, bracelets and earrings.
The traditional accompanying male dress is a dark suit, black necktie and shoes.
http://en.wikipedia.org/wiki/File:Mantilla.jpeg