As far back as records go, Nowruz has been, either in fact or by intention, a celebration of early spring, when the sun begins to regain strength and overcome winter’s cold and darkness and when there is a renewal of growth and vigour in nature. Zoroastar’s people were demonstrably animatists (M. Boyce, 1992, pp. 53-5), that is, they apprehended a cognitive spirit, mainyu (M. Schwartz, p. 641), in all things, tangible or intangible. So for them this return of spring would have represented an annual victory for the Spirit of the sun; and Zoroaster saw in it also, it appears, the symbol of a still more glorious victory to come. This was the especial hope which he offered his followers (see FRAŠEGIRD), that the present struggle between good and evil on all planes, physical, moral and spiritual, will end in total victory for the good. Our “limited time” will then be succeeded by the “Time of Long Dominion” (virtually eternity), with the world and all that is in it restored to the perfect state in which it was created by Ahura Mazdā. A traditional spring festival, ushering in the loveliest season of the year with joyous festivities, could thus, be renamed the “(festival of the) New Day” and celebrated with religious rites, be a recurrent reminder of the unique “New Day” which will eventually bring everlasting bliss; and so this observance could aid faith and deepen understanding of doctrine. This is likely to have been a way of teaching to which Zoroaster naturally resorted, preaching as he did to an ancient, non-literate, pastoral people (see AVESTAN SOCIETY), who used no images to sustain belief, but venerated divinity in and through what they saw and experienced in the world around them.
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The Wiki entry on Norwuz is here.