The thirteenth day of the Iranian New Year festival is Sizdah Bedar (meaning "thirteen outdoors"), is a day of festivity in the open, often accompanied by music and dancing. The day is usually spent at family picnics.
The thirteenth day celebrations stem from the belief of the ancient Persians that the twelve constellations in the Zodiac controlled the months of the year, and each ruled the earth for a thousand years. At the end of which, the sky and the earth collapsed in chaos. Hence, Norooz lasts twelve days and the thirteenth day represents the time of chaos when families put order aside and avoid the bad luck associated with the number thirteen by going outdoors and having picnics and parties.
At the end of the celebrations on this day, the sabzeh grown for the Haft Sin (which has symbolically collected all the sickness and bad luck) is thrown into running water to exorcise the demons (divs) from the household. It is also customary for young single women to tie the leaves of the sabzeh before discarding it, so expressing a wish to be married before the next year's Sizdah Bedar.