The quinquennial Neronia was a massive Greek-style festival created by the Roman Emperor Nero. The festival was in three parts. The first was music, oratory and poetry, the second was gymnastics, and the last was riding. It was held twice—once in AD 60 and once in AD 65.
Timing of the quinquennial Neronia has puzzled historians. "Quinquennial" means "every five" but in practice means every four. Romans counted things inclusively. So between 1 and 5, there are 4 years. Additionally, being a Greek festival, a four-year cycle is consistent with customs.
Yet, the quinquennial Neronia took place in AD 60, 6 years after Nero's rise to power in AD 54, and then again in AD 65, 5 years after that.
The quinquennial Neronia is mentioned in Suetonius, The Lives of Twelve Caesars Life of Nero 12 and The Lives of the Poets Life of Lucanus, Tacitus, Annals XIV.20 and XVI.2, and Cassius Dio, Roman History LXI.21.