Louisiana holds its primary election on November 6, 2012 when all the other states hold their general elections. Louisiana then has a runoff/general election on December 8, 2012.
The primary system is open (similar to Washington and California) meaning it allows all candidates, regardless of party, to run against each other on the same primary ballot. So, all congressional and state level candidates will run on Nov. 6, and if no candidate captures more than 50% of the vote, the top two vote-getters (regardless of party) advance to the runoff election.
Why does Louisiana take this approach? To save taxpayers money. By holding one large statewide election, and then a second smaller election that could be held in some parishes and not others depending on the winner margins, Louisiana saves the taxpayers money. If all races have a candidate break the 50% threshold, no runoff is held and Louisiana spends even less money.
There is one congressional district that has two incumbent Republicans paired against each other. If neither candidate receives at least 50% of the vote on Election Day, we may not know the complete member list of the 113th Congress until early-mid December.