Mehr is an ancient Iranian term meaning friendship and affection. Other meanings are sunshine, sun, and in Persian language equivalent of Avestan Mithra, meaning "covenant" and the Zoroastrian divinity from whom kindness and mercy is requested and who is also associated with the sun and friendship (www.wikipedia.org).

Before appearance of Zoroaster, Mehr was the name of one of the gods, known as the god of sun.
The seventh month of the year and the sixteenth day of the month of the Zoroastrian calendar are dedicated to Mehr and named after him. This practice is also evident in the Iranian calendar, which derives its month names from the Zoroastrian ones (www.wikipedia.org).

Iranian people used to celebrate sixteenth to twenty first of Mehr, when the day-name and the month-name are the same, and this festival was called Mehregān .

In ancient Persia Mehregān was celebrated in an extravagant style at Persepolis. Not only was it the time for harvest, but it was also the time when the taxes were collected. Visitors from different parts of the the Persian Empire brought gifts for the king all contributing to a lively festival. In a non-Zoroastrian context, where Mehr/Mithra is no longer worshiped, Mehregān still remains a celebration amongst family and friends, but today it is recognized as a harvest festival. The festival symbolically ends with bonfires and fireworks (www.wikipedia.org).