The thirteenth day of the new year festival is Sizdah Bedar (literally meaning "passing the thirteenth day", figuratively meaning "Passing the bad luck of the thirteenth day"). This is a day of festivity in the open, often accompanied by music and dancing, usually at family picnics.
Sizdah bedar celebrations stem from the ancient Persians' belief 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 earth collapsed in chaos. Hence Nowruz 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 Seen (which has symbolically collected all 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. Another tradition associated with this day is Dorugh-e Sizdah, literally meaning "the lie of the thirteenth", which is the process of lying to someone and making them believe it (similar to April Fools Day).
Other articles related to "sizdah, sizdah bedar":
... Sizdah Berdar is reported to have been celebrated by the Iranians lived on the Iranian Plateau as far back as 536 BC ... It is puzzling however to see no record about Sizdah Bedar after Iran became a part of the Muslim World in the seventh century ... (1794–1925) referred to the celebration of Sizdah Bedar on their Itinerary Reports (in Persian Safar Naameh-haa) ...
... The Persian texts of a few poems on Sizdah Bedar composed by contemporary Iranian poets may be viewed online on the Chain of Poems on Sizdah Bedar ... composed by Sohrab Sepehri in which he refers to the myth of Sizdah Bedar as related to the Angel of Rain ...