Eating twelve grapes (doce uvas) as the clock strikes midnight is a New Year's Eve tradition all over Spain. But as I explain in my post on Twelve Grapes (doce uvas), you must follow the rules to ensure good luck. The grapes should be green. You should eat one grape with each chime of the clock. It's especially important to have the twelfth and last grape in your mouth the second it turns midnight. You may also wish to observe other elaborations to the ritual, such as standing on your right foot as you eat the grapes. (That way you'll enter the new year on the right foot). Again, see Twelve Grapes (doce uvas) for more information.
It's best to use seedless grapes for obvious reasons. You may also want to peel the grapes, which makes it easier to gobble twelve in a row in less than half a minute.
In Spain, the clock everyone watches is in the Puerta del Sol in Madrid.
You can watch the Puerta del Sol party live on-line at Rtve.es la 1 en directo. Of course the problem is the broadcast won't be at midnight outside of Spain's time zone. (The Puerta del Sol celebration comes on at six in the afternoon here in Florida, Eastern Standard Time). So we watch the big ball in Times Square, which creates some timing challenges because the countdown is done by seconds, not chimes.