There are a number of accounts for the origin of the 13th. The popular belief is that since the number 13 is ill-omened, people spend it away from home to turn bad luck away from it.
Another account which is more scientific says that as old Iranians celebrated the first 12 days of the New Year representing the 12 months, they held the 13th special as the official ending to the celebrations.
Yet another mythological account goes that the son and daughter of Gayomart, the first man and father of all humans in Persian mythology, were engaged in the 13th day of the year. That is why people hold the day as one of greenery and fertility. The account continues that as the two got engaged, they knotted myrtaceae twigs, which is an occasion observed today by some when getting married.