Sizdah Be-dar is one of the Nowruz celebrations. It is celebrated on the 13th day of Farvardin (the first month of Iranian calendar).
In general, the celebration of Sizdah Be-dar is a bit ambiguous among Iranian celebrations; because there is no beginning or basis for it like the other Iranian festivals. There is no direct mention of such a celebration in historical books. But there are some references to it in some ancient sources.
Ancient Iranians celebrated 13th day of Nowruz after celebrating the first twelve days of the year (attributed to the twelve months of the year). Sizdah be-dar is a a glorious day. Iranians went to plain fields and drank wine and danced together on this day; and that’s how they officially ended Nowruz.
In the ancient stories, we read that the 13th of Farvardin, is the day that the God of rain won the battle against the demon of drought. To help the god of rain winning this battle, every year, people go to the rivers and plains to pray. After defeating the demon, people started dancing and singing to celebrate this victory. This victory brings another year of blessing for the nature.
It is known that the word is attributed to the belief that people have :”We should go out on this day, because this day is malevolent (due to some stories from many years ago); so we are sending it away from ourselves“. In Persian sending something away is: “Dar” and as it is the 13th day of the month, they call this day: “Sizadah Be dar“.
But when we look at the meanings of words, there is also another interpretation of the word.
The word “Dar” can be the abbreviation form of “Dar o Dasht” (Means: Plain field). On the other hand the word “Be” means going towards something or somebody in Persian; and we can also get the meaning “Going to the plain fields and nature on the 13th day” from the combination of “Sizdah (13) + Be + Dar“.