May 2016 - Some time after the beginning of Zoroastrianism, probably starting under the Persian Empire around 500 BC, Zoroastrian worshippers began to celebrate the first day of spring, which was the first day of their New Year, as an important holiday. It's probably related to an even older Babylonian New Years tradition, which is celebrated in the Enuma Elish, and probably goes back to before there was writing, before 3000 BC.
People knew when the first day of spring was because their 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 - in Persian times the lords brought gifts 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 including a little lamp with a flame, sugar, flowers, rice, rose water, and betel nut. Sometimes they put out decorated eggs, a bowl with fish in it, or coins. Nowruz lasted for twelve days, and 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.
(Compare Nowruz to the older Babylonian New Year, to the Jewish holidays of Purim and Passover, the Chinese holiday of Qingming Jie, and the Christian holiday of Easter, which all happen at the same time of year, and to 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.
Or check out this website about Zoroastrianism.