The tradition consists in eating a grape with each bell strike at midnight of December 31. According to the tradition, that leads to a year of prosperity. In some areas, it is believed that the tradition wards away witches and general evil. Each grape is eaten with each beat of the bell.
The twelve grapes are linked to the Puerta del Sol tower clock, where this tradition started and from where the change of year is always broadcast.
This tradition was adopted also by places with a broad cultural relation with Spain such as Mexico and other Latin American countries, as well as Hispanic communities in the United States. Countries as far as the Philippines have adopted the tradition.