His feast day is October 4th and he was designated the patron saint of ecology and the environment by Pope John Paul II.
His early life was certainly not that of a saint and he enjoyed partying and drinking. However after a battle, he spent a year in prison during which he started to receive visions from God. After release from prison, he heard God telling him to repair the christian church and live a life of poverty. He felt a great bond with nature and animals and considered them part of his brotherhood.
The following prayer is ascribed to him - composed while recovering from illness and called the Canticle of the Sun.
Most high, all powerful, all good Lord!
All praise is Yours, all glory, all honor, and all blessing.
To You, alone, Most High, do they belong.
No mortal lips are worthy to pronounce Your name.
Be praised, my Lord, through all Your creatures,
especially through my lord Brother Sun,
who brings the day; and You give light through him.
And he is beautiful and radiant in all his splendor!
Of You, Most High, he bears the likeness.
Be praised, my Lord, through Sister Moon and the stars;
in the heavens You have made them bright, precious and beautiful.
Be praised, my Lord, through Brothers Wind and Air,
and clouds and storms, and all the weather,
through which You give Your creatures sustenance.
Be praised, my Lord, through Sister Water;
she is very useful, and humble, and precious, and pure.
Be praised, my Lord, through Brother Fire,
through whom You brighten the night.
He is beautiful and cheerful, and powerful and strong.
Be praised, my Lord, through our sister Mother Earth,
who feeds us and rules us,
and produces various fruits with colored flowers and herbs.
Be praised, my Lord, through those who forgive for love of You;
through those who endure sickness and trial.
Happy those who endure in peace,
for by You, Most High, they will be crowned.
Be praised, my Lord, through our sister Bodily Death,
from whose embrace no living person can escape.
Woe to those who die in mortal sin!
Happy those she finds doing Your most holy will.
The second death can do no harm to them.
Praise and bless my Lord, and give thanks,
and serve Him with great humility.