A good place to start is in early December of 62 at the festival of the Bona Dea (Good Goddess), where Roman matrons would come together to sacrifice to the goddess. One of the basic tenets of this festival was that only women were allowed to attend. This particular year, the festival was held in the house of the Julius Caesar, who was praetor elect.
All was well, until Clodius was found to have broken into the festival dressed up as a woman. When a slave girl detected him, he fled. As was typical for the Romans, when this violation came to light, there was an instauratio. According to Cicero, the Vestal Virgins repeated the sacrifice to the goddess immediately. But there was one ex-praetor, who would not allow the matter to be forgotten.
Q. Cornificius took it upon himself to bring the matter up to the senate for further discussion. Had it been not for Cornificus, this issue would have probably been forgotten in the annals of history.The upshot of all of this was twofold. First, the senate found this issue important enough to be referred to the College of Pontiffs and the Vestals.Second, the decision of the Pontiffs and the Vestals was that Clodius’ actions were officially deemed to be nefas and so they decreed that another instauratio ought to be undertaken.