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.[a] The last of the Tarquins possessed the Roman nation, an unjust man, but nevertheless strong in war.[b]
Plutarch holds that the rex sacrorum was a substitute for the former king of Rome here as 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. The "flight of the king" was the swift exit the proxy king was required to make from that place of public business. 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.