The earliest historical records of Nowruz celebrations trace back to the Achaemenid Empire.
Evidence for a spring New Year celebration appears in accounts connected with the Achaemenid Empire, which ruled much of the Iranian world from the sixth to the fourth century BCE. The Greek historian Xenophon described a celebration at Persepolis, the Achaemenid ceremonial capital, where the empire’s subject peoples presented gifts to the King of Kings. This courtly ceremony is widely connected with the tradition later known as Nowruz.
The evidence requires some care. The word “Nowruz” itself is not preserved in surviving Achaemenid inscriptions, and the exact origins of the festival may reach further back into Iranian religious and seasonal traditions. The Achaemenid record is therefore the earliest major historical evidence, not proof that the celebration suddenly began under one particular king.
Later dynasties expanded and documented the festival. Extensive descriptions survive from the Sasanian period, when Nowruz became the most important royal holiday and customs such as audiences, gifts, and pardons were associated with it. The Median, Parthian, and Sasanian empires are important to Iranian history, but the earliest recorded association in this question is Achaemenid.