Sizdeh Bedar is celebrated on this 13th day of the Persian new year
Sizdah Be-Dar (also romanized as Sizdahbedar and Sizdah Bedar, frequently stylized as "13 Bedar") (Persian: سیزده به در, literally: 13th in outdoors) is a festival in the Iranian culture, and part of the Nowruz new year celebration rituals, held on the 13th of Farvardin (the 1st month of the Iranian calendar), during which people spend time picnicking outdoors.
Sizdah Bedar is the day Tir (The Blessed day) of the month Farvardin from ancient Persian (Iranian) calendar, which was the first day of agricultural activity in ancient Persia. Be-dar in Persian means going out. In this day Iranians go to have fun with their families all the day long. In some sub cultures they have 14 Be-Kaar, which means 14 to work. During recent years, Jewish-Islamic people have related 13 Be-dar as the unlucky 13, which isn't related to ancient Persian culture, one of the reasons for such interpretation is that in some years, 13 Farvardin is the same as April 1 (April Fools' Day).
Sizdeh Bedar is celebrated primarily in Iran with the largest groups of celebrants. Smaller groups of people in countries such as Armenia, Turkey, and Central Asia also observe Sizdeh Bedar. An increasing number of participants are taking part in the holiday. In cities like Los Angeles, with large populations of Iranians, a growing number of parks are set up by cities to accommodate the large numbers of celebrants.