St. Francis of Assisi, patron saint of animals and environment could be viewed as the original Earth Day advocate. Francis' devotion to God was expressed through his love for all of God's creation. St. Francis cared for the poor and sick, he preached sermons to animals and praised all creatures as brothers and sisters under God.
St. Francis of Assisi, patron saint of animals and ecology, was born in Italy around 1181-1182.
After a wild youth and a brief career as a soldier, Francis had a conversion experience that inspired him to renounce his family’s wealth and devote his life to God.
His dedication to poverty, humility, obedience, patience and compassion soon attracted followers and, in 1209, he received permission from Pope Innocent III to form a new religious order known as the Friars Minor (commonly called the Franciscans).
Stories about St. Francis and animals
The earliest books about Saint Francis were written by Thomas of Celano, a member of Francis' religious order who knew the saint personally. Thomas' books reveal Francis' love for animals and his conviction that religious faith and care for God's creatures go hand-in-hand.
Preaching to the birds
"One time as [Francis] was passing through the Spoleto valley, he came upon a place near Bevagna, in which a great multitude of birds of various kinds had assembled. When the holy one of God saw them, because of the outstanding love of the Creator with which he loved all creatures, he ran swiftly to the place. He greeted them in his usual way, as if they shared in reason. As the birds did not take flight, he went to them, going to and fro among them, touching their heads and bodies with his tunic.
"Meanwhile his joy and wonder increased as he carefully admonished them to listen to the Word of God: 'My brother birds, you should greatly praise your Creator and love Him always. He clothed you with feathers and gave you wings for flying. Among all His creatures, He made you free and gave you the purity of the air. You neither sow nor reap, He nevertheless governs you without your least care.'
"At these words, the birds gestured a great deal, in their own way. They stretched their necks, spread their wings, opened their beaks and looked at him. They did not leave the place until, having made the sign of the cross, he blessed them and gave them permission. On returning to the brothers he began to accuse himself of negligence because he had not preached to the birds before. From that day on, he carefully exhorted birds and beasts and even insensible creatures to praise and love the Creator."
Read more of Thomas of Celano's stories [PDF] about St. Francis and animals.
—From Thomas of Celano, The Treatise on the Miracle of Saint Francis, (1250-1252), ed. Regis J. Armstorng, OFM Cap, J.A. Wayne Hellmoann, OFM Cov, William J. Short, OFM, The Francis Trilogy of Thomas of Celano (Hyde Park: New City Press, 2004), 329-330.
The Canticle of the Creatures
Although Francis left behind relatively few written documents, his Canticle of the Creatures (also known as The Canticle of Brother Sun) is included within the Catholic Catechism and was considered by Pope John Paul II to be a beautiful reminder that everything from God bears significance.
Most High, all-powerful, good Lord,
Yours are the praises, the glory, the honor, and all blessing.
To You alone, Most High, do they belong,
and no man is worthy to mention Your name.
Praised be You, my Lord, with all your creatures,
especially Sir Brother Sun,
Who is the day and through whom You give us light.
And he is beautiful and radiant with great splendor;
and bears a likeness of You, Most High One.
Praised be You, my Lord, through Sister Moon and the stars,
in heaven You formed them clear and precious and beautiful.
Praised be You, my Lord, through Brother Wind,
and through the air, cloudy and serene, and every kind of weather
through which You give sustenance to Your creatures.
Praised be You, my Lord, through Sister Water,
which is very useful and humble and precious and chaste.
Praised be You, my Lord, through Brother Fire,
through whom You light the night
and he is beautiful and playful and robust and strong.
Praised be You, my Lord, through our Sister Mother Earth,
who sustains and governs us,
and who produces varied fruits with colored flowers and herbs.
Praised be You, my Lord, through those who give pardon for Your love
and bear infirmity and tribulation.
Blessed are those who endure in peace
for by You, Most High, they shall be crowned.
Praised be You, my Lord, through our Sister Bodily Death,
from whom no living man can escape.
Woe to those who die in mortal sin.
Blessed are those whom death will find in Your most holy will,
for the second death shall do them no harm.
Praise and bless my Lord and give Him thanks
and serve Him with great humility.
—From Francis of Assisi, "The Canticle of Brother Sun," Regis J. Armstrong, OFM CAP and Ignatius C. Brady, OFM, trans., Francis and Clare: The Complete Works (New York: Paulist Press, 1982), 38-39.
The Feast of St. Francis and the Blessing of the Animals
Every year on or around October 4, Christians throughout the world celebrate the Feast of St. Francis with prayers for creation and a blessing of the animals. Services in the United States currently occur in 47 states and the District of Columbia. 
St. John the Divine
"Blessings of the animals/pets entered the mainstream U.S. cultural scene in the latter part of the twentieth century. Although blessings of various animals took place in different times and places, and for varying reasons, throughout Christian history, the growth of these rituals beginning in the late twentieth century is dramatic [...] By the early twenty-first century, hundreds, if not thousands, of these blessings take place annually.
"The Cathedral of St. John the Divine in New York City [...] epitomizes the idea and, some might contend, the ideal of the inclusion of animals in the Christian worship setting. The liturgical event is officially titled the 'Feast of St. Francis' and includes the 'Holy Eucharist and Procession of Animals' presided over by the bishop of New York [...]"
"Hopeful participants arrive as early as 7 a.m. to wait for the 11 a.m. worship service to begin because the cathedral fills to at least its 3,500-person, standing-room-only capacity for the Feast of St. Francis ritual. Many bring animal companions with them. On one occasion, Chuckles (the parrot), Philly and Nicholae (dachshunds), and Jack the bulldog stood among humans, ferrets, cats, turtles, and numerous canines, just to name the most prominent species, in a line that spreads several city blocks [...]"
“Following (a) […] prayer the bishop blessed all the animals […] ‘Live without fear: your Creator loves you, made you holy, and has always protected you. Go in peace to follow the good road and may God’s blessing be with you always. Amen.”
—From Laura Hobgood-Oster, Holy Dogs & Asses: Animals in the Christian Tradition (Urbana and Chicago: University of Illinois Press, 2008), 106-116.
Examples of prayers for animals at other blessing services
"Hear our humble prayer, O God,
for our friends the animals, your creatures.
We pray especially for all that are suffering in any way:
for the overworked and underfed,
the hunted, lost, or hungry;
for all in captivity or ill-treated,
and for those that must be put to death.
For those who deal with them,
we ask a gentle heart of compassion,
gentle hands, and kindly words.
Make us all true friends to animals
and worthy followers of our merciful Savior, Jesus Christ.
—From United States Conference of Catholic Bishops, "Prayers and Blessings for Special Occasions: Saint Francis: Blessing of Animals"
"The animals of God's creation inhabit the skies,the earth and the sea.
They share in the fortunes of human existence
and have a part in human life.
God, who confers gifts on all living things,
has often used the service of animals
or made them reminders of salvation ...
We, therefore, invoke God's blessing on these animals.
As we do so, let us praise the Creator
and thank God for setting us as stewards
over all the creatures of the earth."
—From "Occasional Services: A Service for the Blessing of Animals," The United Methodist Church Book of Worship (Nashville: The United Methodist Publishing House, 1992), 608-609.
 Catechism of the Catholic Church, Part One, Section Two, Chapter One, Article 1, paragraph 5, subsection II, line 344.
 See Ioannes Paulus PP. II, "Litterae Apostolicae Inter Sanctos." See also John Paul II, "Address of Pope John Paul II to the Second International Meeting 'Young People to Assisi'",Aug. 18, 2001.
 Figures obtained from American Catholic, "St. Francis of Assisi: Pet Blessings 2008."