Varro and Ovid traced the observance to the flight of the last king of Rome, Tarquinius Superbus, in 510 BC.
In his Fasti, Ovid offers the longest surviving account of the observance: Now I must tell of the flight of the King, six days from the end of the month.
[b] Plutarch holds that the rex sacrorum played as a substitute for the former king of Rome in various religious rituals.
The rex held no civic or military role, but nevertheless was bound to offer a public sacrifice in the Comitia on this date.
It may be that the two versions are to be reconciled by taking the "flight" of the rex sacrorum as a reenactment of the expulsion of Tarquinius.