13 Bedar (read: Sizdah Bedar) It's the holiday celebrating the end of Nowrooz (Persian new year) on which everybody goes out to a park and spends a great day outdoors in the nature. It can be translated as "getting rid of thirteen" which traditionally is considered a number bringing the bad luck.
A ritual performed at the end of the picnic day is to throw away the Sabzeh( grass grown at home prior to 13 bedar) from the Norooz Haft Seen table. The sabzeh is supposed to have collected all the sickness, pain and ill fate hiding on the path of the family throughout the coming year! Touching someone else's sabzeh on this thirteenth day or bringing it home is, therefore, not a good idea and may result in inviting their pain and hardship to oneself.