Madrid is the city where the Puerta del Sol clock is used for Spain's televised New Year's Eve countdown.
Puerta del Sol is a central square in Madrid and the setting for the country's most famous public New Year's celebration. At midnight, television viewers follow the clock's twelve chimes and eat one grape with each chime. The ceremony is especially associated with the clock mounted at the former post office building, now the headquarters of the Community of Madrid.
The square's event has become a national reference point even for people celebrating elsewhere. Families across Spain often watch the broadcast while preparing their twelve grapes, turning a local clock into a shared nationwide countdown.
The “ball” in this question refers to the clock's visible timekeeping feature, not the illuminated sphere dropped in New York's Times Square. Madrid's tradition is based on twelve chimes and twelve grapes. Puerta del Sol also has a famous statue of the Bear and the Strawberry Tree, a symbol of Madrid, but that landmark is unrelated to the countdown mechanism.