Zoroastrianism became much more popular suddenly when the Persian kings became Zoroastrians around 550 BC. Soon after that, Zoroastrian worshippers began to celebrate the first day of spring as an important holiday. That was the first day of their New Year. Nowruz is probably related to an even older Babylonian New Year’s tradition. The Enuma Elish celebrates this earlier New Year, which probably goes back to before there was writing, before 3000 BC.
Iranians knew when the first day of spring was because Iranian astrologers kept track of the movements of the stars and the sun. When the sun leaves Pisces and enters the constellation of Aries, that’s the New Year – Nowruz.
People celebrated Nowruz by plowing the first furrows in their fields. They exchanged gifts. Under the Persian Empire, rich lords brought gifts of eggs to the Persian king. People cleaned out their houses and got new clothes. They put out a special tray with seven symbols of the season. The seven symbols included a little lamp with a flame, sugar, flowers, rice, rose water, and betel nut. Sometimes people put out decorated eggs, a bowl with fish in it, or coins.
Nowruz lasted for twelve days. People visited their families and friends. Some people built bonfires and jumped over them for good luck. Children trick-or-treated from door to door, or left baskets outside houses to be filled with candies and nuts.
People ate special foods for Nowruz too. They ate green spring herbs like parsley, spinach and cilantro, stuffed grape leaves or dolmades, fish, baklava and candied almonds, and dried fruits mixed with nuts.
Try comparing Nowruz to the older Babylonian New Year, to the Jewish holidays of Purim and Passover. See how Nowruz is also like the Chinese holiday of Qingming Jie, and the Christian holiday of Easter, which all happen at the same time of year, and like Chinese New Year, which is just a little earlier.
Zoroastrianism, by Paula Hartz (updated 2004). Easy reading.
The Usborne Book of World Religions, by Susan Meredith (1996). Easy reading.
An Introduction to Ancient Iranian Religion: Readings from the Avesta and Achaemenid Inscriptions, by William Malandra (1983). A nice clear explanation of ancient Zoroastrianism, by a specialist, for adults.