'Thirteen outdoor') also known as Nature's Day (روز طبیعت – rūz-e tabī'at),[citation needed] is an Iranian festival held annually on the thirteenth day of Farvardin, the first month of the Iranian calendar (and the first month of spring, during which people spend time picnicking outdoors.

[3] Generally, among Iranian festivals, "Sizdah Bedar" is a bit vague in terms of historical roots.

There is no direct mention of such a celebration in history books prior to Qajar era.

It was an official ending to Nowruz and a beginning to the remaining twelve months of the year.

Nature Day is the ancient Persian tradition of celebrating the occasion of the god of rain's victory over the drought-inducing demon.

In ancient Iran, after celebrating the 13th day of Nowruz, which was the beginning of an agricultural semi-year, Iranians went to their fields and farms or plains and deserts and the banks of rivers to worship.

A custom performed at the end of the picnic is to throw the sprouted greens (سبزه – Sabze) that were part of the traditional Haft-Seen Nowruz display into moving water.

[8] It is a common dish found in many regions and cultures within West Asia, including Iran, Azerbaijan, and Turkey.