Complex sentence: There is an annual air show for the remembrance of War Heroes at the beginning of the Iranian New Year (Nowruz): hold from March 21 to 4 April, it is the only military Air show in Iran.
Split into simpler sentences: There is an annual air show for the remembrance of war heroes at the beginning of the Iranian New Year (Nowruz). Held from March 21 to April 4, it is the only military air show in Iran.