"Almost all Iranians, no matter their religion, language and race, celebrate Yalda," said Hooshang Sohaei as he stood in a long line at a confectionary shop in north Tehran to buy sweets and dry fruit.
Zoroastrianism's central theme is the struggle between the good spirit Ahura Mazda and the evil Ahriman. Yalda, marked on the winter solstice, recognizes the symbolic victory of light over darkness as day time starts growing longer and nights become shorter.
In the streets of the capital, Tehran, fruit vendors enjoyed their busiest day of the year, and confectionaries were packed with customers buying up provisions for the feast.
Traditionally, families and friends sit around a furnace and elders recite tales or read poetry, often from the Shahnameh, an ancient epic by Iran's greatest storyteller, Abolqasem Ferdowsi, as they drink tea and eat nuts and fruit.
Others debate the latest domestic and international developments, which these days means the economic crisis, Iran's nuclear program and the recent shoe-throwing incident involving President George W. Bush and an angry Iraqi journalist.
Wanting to have stock to sell at Yalda, fruit vendors even keep watermelons and other fruit from the summer harvest refrigerated until the winter feast.
The national celebration, like several other pre-Islamic holidays, has survived the advent of Islam and efforts after the 1979 Islamic revolution by hard-line clerics to discourage such festivals as un-Islamic.
Opposition to Yalda, however, is mild because of its emphasis on family gatherings, a value promoted by Iran's ruling clerics.
"In the industrialized world of today, Yalda is the way to bring love and happiness to families," said Maryam Firouzi, a housewife. "It gives people the chance to meet and put aside their differences."
Zoroastrianism lost dominance in Iran after Muslim Arabs conquered Persia in the 7th century. Today, most of Iran's 70 million people are Shiite Muslims.
But some 60,000 Zoroastrians remain today - dwindled down from 300,000 in the 1970s, when many emigrated to the United States. Iran also has small Christian and Jewish communities.
Since the 1979 revolution, the Islamic Republic has tolerated Zoroastrians, giving the community official status and allowing members to practice their rites. Human rights reports say Zoroastrians - like Iran's Jews and Christians - suffer some discrimination and are kept out of some jobs.