September 1 coincides with the beginning of the Eastern Orthodox New Year.
In 2015, Pope Francis established the “World Day of Prayer for the Care of Creation,” to be celebrated annually on September 1.
This date was chosen to coincide with a day that Eastern Orthodox Christians already had on their liturgical calendar.
Pope Francis explains this in his letter that established this celebration in the Catholic Church.
The celebration of this Day, on the same date as the Orthodox Church, will be a valuable opportunity to bear witness to our growing communion with our Orthodox brothers and sisters. We live at a time when all Christians are faced with the same decisive challenges, to which we must respond together, in order to be more credible and effective. It is my hope that this Day will in some way also involve other Churches and ecclesial Communities, and be celebrated in union with similar initiatives of the World Council of Churches.
In 1989, the late Ecumenical Patriarch Demetrios I, wrote a letter on the environment, marking the first annual “Day of Prayer for Creation” on September 1.
This day was chosen for a specific reason for Orthodox Christians. It is traditionally the first day of the new liturgical year in the Orthodox church.
According to the Orthodox Church in America, “Before the introduction of the Julian calendar, Rome began the New Year on September 1.”
The beginning of a new year reflects the symbolism of new life as well as the themes of harvest, which commonly occurs in September in the Northern Hemisphere.
It also coincides with a day that has traditionally been associated with Jesus’ entering the synagogue to announce his mission to preach the Gospel.
And he stood up to read; and there was given to him the book of the prophet Isaiah. He opened the book and found the place where it was written,
The Spirit of the Lord is upon me,Luke 4:17-19
because he has anointed me to preach good news to the poor.
He has sent me to proclaim release to the captives
and recovering of sight to the blind,
to set at liberty those who are oppressed,
to proclaim the acceptable year of the Lord.”
It is in this context that the World Day of Prayer for the Care of Creation was established in the Catholic Church.