Complex sentence: The basis of nearly all of Iranian national festivals are from its Pre-Islamic Zoroastrian era, however there are some festivals that are celebrated exclusively by Zoroastrians and some with less extent in other communities too.
Split into simpler sentences: The basis of nearly all of Iranian national festivals are from its Pre-Islamic Zoroastrian era. However, there are some festivals that are celebrated exclusively by Zoroastrians and some with less extent in other communities too.