TEHRAN, April 2 (Xinhua) -- Iranians celebrated the 13th day of the spring festival, officially called Nature Day, in green environments on Monday.

Many big cities in Iran looked empty and unpopulated as most of the Iranians moved out of their houses, heading for parks, gardens or countryside to enjoy the company of friends and family members in picnic.

As a national tradition since ancient times, people threw away the green sprouts, known as Sabzeh, which had been prepared for the first day of new year on March 21.

Another popular tradition of the occasion is knotting blades of grass by young unmarried girls hoping to marry soon and to express their wish for good fortune in life and love.

While the young girls were singing and knotting the blades of grass, the young boys played games and sports.