@burcidi-- I saw a news report on Iran the other day that talked about religion. It said that Iran is almost 100% Muslim. Zoroastrians are 1% but along with some other groups like Jews and Christians.
I don't know whether Sadeh is a national holiday in Iran but the program did mentioned Newroz celebrations. I don't see why they would celebrate the Zoroastrian new year and not Sadeh. Maybe someone more informed on this than I can comment on it.
What I ultimately gathered from the TV program about Iran, religion and celebrations is that even though Iran is an Islamic country, they have not stopped following their ancient traditions and holidays from before the arrival of Islam. So they celebrate both Islamic holidays and also historically Persian (and Zoroastrian) traditions like Newroz.