The ۱۳th day of the Persian New Year is called Sizdah Bedar, also referred to as Nature Day, and usually falls on April ۱

The 13th day of the Persian New Year is called Sizdah Bedar, also referred to as Nature Day, and usually falls on April 1. Pranks have reportedly been played on this holiday since 536 BC, making it perhaps the oldest known joke day.
The festival of Sizdah Be-dar also has its roots in Zoroastrianism. Zoroastrians believe that the joyous celebration of this holiday helps cleanse all bad thoughts.
People traditionally spend the day outside picnicking and playing games. After the picnic, you throw away all green vegetables, which traditionally represent potential illness or bad luck for the coming year.

@Generalenglish