Contrary to popular myth, number thirteen does not demonstrate bad omen in Persian culture, it is merely chosen as a day to celebrate.
Based on traditional belief the thirteenth day of every month belongs to Tishtrya, the god of rain, Zoroastrian benevolent divinity associated with life-bringing rainfall and fertility. According to Zoroastrians, in order to have the god of rain as victorious and the fiend of drought as destroyed in the New Year, people should commemorate Tishtrya and ask him for rain.
Therefore on the last day of Nowruz festival and when the earth grows green, people leave their houses for water streams on the 13th to ask Tishtrya for rain. Feasting on traditional foods, munching nuts and playing group games are inseparable ingredients of the happy occasion.
Knotting grass blades and wishing upon the knot is another popular tradition of this day. Once the knot is tied the grass is thrown into water stream. It is believed if the knot is opened, fortune finds the way and wishes will come true. Some people also pull practical jokes and tell lies on this day, calling it the Thirteenth Lie, which is a tradition similar to April Fools. People will also release goldfish into a pond or river – a symbol of freedom. Iranian families all eat alfresco, preferably near water springs and lush greener, on this day.
Sizdah-Bedar is the last day of New Year holidays. On the following day, routine life resumes; schools and offices open after 14 days and life heads back to normalcy.