This is a must if you can see it, not run to 'jockey club' rules so expect to see some heavy encouragement on the part of the 'jockeys'.
How ever it's traditional, to get more info go to <www.balisweethome.com/en/bali/tradition/mekepung.htm>
It's called 'mekepung', the page you will bring up is showing race dates for 2004 but if you go into the general search engines i'm sure you will find out when the 2005 dates are. Quite often they have practise days too. Expect to be there early in the morning though, so if your down south it's an early start.
Don't forget it's traditional so try to see it if only once, enjoy the day.