The Spanish Christmas lottery, El Gordo or 'The Fat One', is the biggest lottery in the world and also one of the oldest, having started in 1812. While the first prize in 2005 was 'only' 3€ million, El Gordo has the biggest prize pool of any lottery in the world, last year totaling over two billion euros!
 
It is drawn every December 22, and the process is long, taking in excess of three hours, so you will likely see people sat by their radios for much of the day, waiting for the results.
 
The Spanish Christmas Lottery works differently to most lotteries in the world. A whole ticket 'billete' is very expensive, costing 200€ in 2005. However, these tickets are split up into ten 'decimos' (tenths) costing 20€ each. When buying your tickets you have the choice of buying a single decimo, a whole ticket, or a fraction of your choice. If you don't buy the whole ticket, someone else will buy the rest of your ticket.
 
So, if you are in Spain before December 22, go into an official 'Loteria Nacional' office - every town will have one. You don't need to buy a whole ticket, but it means that if you win, you'll be sharing your winnings with a stranger!
 
Source: about.com