After carefully studying Sforza's movements, the conspirators made their move on the day after Christmas, 1476, feast day of Saint Stephen, patron saint of Santo Stefano, the church where the deed was to be committed. Supported by about thirty friends, the three men waited in the church for the duke to arrive for mass. When Galeazzo Sforza arrived, Lampugnani knelt before him; after some words were exchanged, Lampugnani rose suddenly and stabbed Sforza in the groin and breast. Olgiati and Visconti soon joined in, as did a servant of Lampugnani's.