In Spain there is a tradition of eating twelve green grapes (doce uvas) on New Year's Eve (Nochevieja). It's supposed to bring good luck, but you can't just lounge around popping grapes into your mouth. (Ensuring good luck is never so easy.) There's a big clock in the Puerta del Sol, one of the central plazas in Madrid. At midnight, the bell of this clock sounds twelve times, and one grape should be eaten for each chime of the bell. See my post Twelve Grapes on New Year's Eve (doce uvas por nochevieja) for more information.
When we're in Spain for New Year's Eve, we (and much of the rest of the country) watch the Puerta del Sol party on television, much the way North Americans watch the New Year's Eve party at Times Square. Instead of waiting for a ball to drop, we listen for the clock tower bell. So in Spain eating the twelve grapes is a straighforward matter, if not exactly easy: just pop a grape into your mouth with each sound of the bell.
When we're in the U.S. for New Year's Eve, eating the twelve grapes can be a bit more complicated. The countdown at Times Square is done by seconds, not clock chimes, and it's difficult (and a bit disgusting) to try to eat twelve grapes in twelve seconds. Although I haven't actually timed the bells of the clock at the Puerta del Sol, I'm pretty sure they don't sound at second intervals. I would say there are two seconds or so between chimes. So we just try to eat the twelve grapes during the last thirty seconds of the year, more or less, and hope for the best.
There are other superstitions connected to the twelve grapes. Esperanza, my mother-in-law, tells me that if I stand on my right foot while eating the grapes, I'll enter the New Year on the right foot. Needing all the help I can get, I always do this.
Esperanza also drops her ring in her flute of champagne or cava for the New Year's toast (brindis). I can't remember why.
But the important thing is to eat twelve green grapes just before midnight and make sure you have the last grape in your mouth as the clock strikes twelve.
For obvious reasons, it's best to use seedless grapes. Many people peel the grapes, to make them easier to eat quickly. Well before midnight, each person should be prepared with twelve grapes--count them--on his or her own plate. This helps to avoid mix-ups and confusion.
Click here to see a picture of the Puerta del Sol clock and more information. The tradition of the twelve grapes is also observed in many other Spanish-speaking countries. Click here to see an article about the twelve grapes at My Latino Voice.