Musa b. Ja'far (ع) speaks about Nawruz - stating it's not part of the Sunnāh the Prophet (ص)
He (ع) says when Al-Mansur asked him to come forth and sit and celebrate this celebration, he said:
I have examined the reports from my grandfather, the Messenger of Allah صلى الله عليه وآله, and I did not find a report for this `eid. And it is a tradition of the Persian and Islam has erased it. And (I seek the) refuge of Allah that we revivify what Islam has erased
Manāqib Al Abi Talib, Ibn Shahr Ashoob, V.4, Pg.318
Nowrūz (Persian: نوروز, meaning “New Day”) is the name of the Iranian/Persian New Year in Iranian calendars and the corresponding traditional celebrations. Nowruz is also widely referred to as the “Persian New Year”.
Nowruz is celebrated and observed by Iranian peoples and the related cultural continent and has spread in many other parts of the world, including parts of Central Asia, Caucasus, South Asia, Northwestern China, the Crimea and some groups in the Balkans.
Nowruz marks the first day of spring and the beginning of the year in the Iranian calendar. It is celebrated on the day of the astronomical Northward equinox, which usually occurs on March 21 or the previous/following day depending on where it is observed. As well as being a Zoroastrian holiday and having significance amongst the Zoroastrian ancestors of modern Iranians, it is also celebrated in parts of the South Asian sub-continent as the new year. The moment the Sun crosses the celestial equator and equalizes night and day is calculated exactly every year and Iranian families gather together to observe the rituals.