In which sporting event does the leader wear a yellow jersey?

Answer 1

Before the days of easy media access, having the different types of top riders wear easily recognizable jerseys was a simple way of helping the spectators track the action.

There's nothing special about the jersey being yellow, it might just as well have been any other easily recognizable colour/pattern - but now they choose yellow.

Answer 2

The Tour de France, a cycle race, the current leader each day wears a yellow jersey the following day.