Sizdah in Persian means “thirteen” which traditionally is symbol of inauspiciousness and unluckiness.
In Persian tradition, sizdah be dar (literally: Good bye thirteenth) is the custom of leaving the house for nature and publicly celebrating the thirteenth (sizdah) day of the month of Farvardin (corresponding to April 2), the last day of the Norouz (Iranian New Year).
This is the last phase of the New Year festival. In modern times people go to parks, suburbs and having a picnic and throw their sabzeh – the seeds they grow near the beginning of Norooz - into a river, symbolizing the cycle of life. Some girls also tie the sprouts of sabzeh on this day, symbolizing their wish for good fortune in life and love. Some people also pull practical jokes and tell white lies on this day, calling it the thirteenth lie (this is very similar to April Fools). People also release goldfish into a pond or river.
It seems our ancestors predicted our industrial living and set up a way for relieving corresponding stresses by seeking refuge to the Nature.