The country best known for the New Year’s Eve tradition of eating twelve grapes is Spain.
In Spain, people eat one grape with each of the twelve clock strikes at midnight on 31 December. Each grape is associated with one month of the coming year, and successfully eating all twelve is traditionally said to bring good luck and prosperity. The custom is known as las doce uvas de la suerte, or “the twelve grapes of luck.”
The most famous public setting is Madrid’s Puerta del Sol, where crowds follow the clock of the Royal House of the Post Office. The midnight chimes are broadcast nationally, so families across Spain often perform the ritual at home while watching television.
The tradition is documented by the late nineteenth century and was strongly popularized in 1909, when grape growers in Alicante promoted it amid a surplus harvest. Portugal and several Latin American countries also have related customs, sometimes using raisins, but Spain is the canonical answer to this question. The grapes are eaten to the clock chimes—not simply at some point during the evening.