New Year's Eve (Nochevieja) in Spain is a party night like everywhere else in the world, though the structure is a little different to in other countries - remember you have to think in SPANISH TIME!
Rather... than starting early and building to a crescendo at midnight, the Spanish see in the New Year sober (well, nearly sober), either with friends or with family, and then go out to the bars at about 12.30. The partying then continues until about 6am (if you fancy an early night) or much, much later, if you don't!
There is an 'ancient' tradition, started by some shrewd farmers about 100 years ago when they were left with too many grapes after the harvest, of eating twelve grapes at the twelve bongs of midnight. This is a fun ritual, only spoiled by the fact that it is almost impossible to buy seedless grapes in Spain - in the rush to chomp down the dozen grapes; everyone ends up biting into a seed and pulling a silly face.
A word of advice: there are four higher-pitched chimes just before the main ones at midnight (known as 'los cuartos') that announce the start of the real ones - make sure you don't start eating your grapes too soon. It catches people out each year - one year a television presenter made the fatal error! For every grape you get right, you will get a month's good luck.