From the Legenda Minor of St. Bonaventure
(de Stigmatibus sacris, 1-4; ed. Quaracchi, 1941; pgg. 202-204)

Two years before Francis, the faithful servant of Christ, gave his soul back to God, he was alone on the top of Mt. Alverna. There he had begun a fast of forty days in honor of the archangel Michael and was immersed more deeply than usual in the delights of heavenly contemplation. His soul became aglow with the ardor of fervent longing for heaven as he experienced within himself the operations of grace.

As he was drawn aloft through ardent longing for God one morning near the feast of the Exaltation of the Cross, and was praying on the mountainside, he saw what appeared as a seraph with six bright wings gleaming like a fire descending from the heights of heaven. As this figure approached in swift flight and came near the man of God it appeared not only winged but also crucified. The sight of it amazed Francis and his soul experienced joy mingled with pain. He was delighted with the sight of Christ appearing to him so graciously and intimately and yet the awe-inspiring vision of Christ nailed to the cross aroused in his soul a joy of compassionate love.

When the vision vanished after a mysterious and intimate conversation it left Francis aglow with seraphic love in his soul. Externally, however, it left marks on his body like those of the Crucified as if the impression of a seal had been left on heated wag. The figures of the nails appeared immediately on his hands and feet. The heads of the nails were inside his hands but on top of his feet with their points extending through to the opposite side. His right side too showed a blood-red wound as if it had been pierced by a lance, and blood flowed frequently from it.

Because of this new and astounding miracle unheard of in times past, Francis came down from the mountain a new man adorned with the sacred stigmata, bearing in his body the image of the Crucified not made by a craftsman in wood or stone , but fashioned in his members by the hand of the living God.

Francis and his followers travelled and preached all over Europe, gaining popularity and followers everywhere. Francis himself preached before the Pope and Cardinals at the Lateran in 1217 and met St. Dominic during that stay in Rome. Lay people were so moved by Francis' preaching that they came to him pleading to join his order, once an entire congregation implored him en masse. At this point he devised his Third Order, which he intended as a middle state for people who weren't ready or able to leave all for the cloister. Many Franciscans also went to preach to the Muslims, Francis himself preached to the sultan as he and his army were faced by crudasers from Europe. Many of the friars who went to Muslim lands were martyred.

Francis' first rule was only approved verbally by Innocent III, and in written form it was overly long and not precise. After some relaxations in the austerity he desired for his friars and imposition of rules by outside or unauthorized people, Francis retired to solitude to entrust an official version of his rule of life to paper. (After he finished a first version it was promptly lost, and Francis was forced to retire again to re-write it.) The rule was pared down from 23 to 12 chapters, and was solemnly approved by Pope Honorius III  on November 29, 1223. It was unique up to that time in that it stressed the vow of poverty, which it made absolute, and in the compromise between the secular and religious states in his Third Order.

Late in his life Francis received the Stigmata,
as told in this account from the Catholic Encyclopedia:

Early in August, 1224, Francis retired with three companions to "that rugged rock 'twixt Tiber and Arno", as Dante called La Verna, there to keep a forty days fast in preparation for Michaelmas. During this retreat the sufferings of Christ became more than ever the burden of his meditations; into few souls, perhaps, had the full meaning of the Passion so deeply entered. It was on or about the feast of the Exaltation of the Cross (14 September) while praying on the mountainside, that he beheld the marvelous vision of the seraph, as a sequel of which there appeared on his body the visible marks of the five wounds of the Crucified which, says an early writer, had long since been impressed upon his heart.

HYMN
Office of Readings

The mysteries of Jesus' passion
Shone forth on Mount Alverna's height,
Where rays of merciful redemption
Diffused their gentle saving light,
As Francis, rapt in contemplation,
The Cross embraced throughout the night.

His fervent prayer in lone seclusion,
His spirit soared to God on high;
The thought of Christ's most bitter passion
Evoked from him a painful sigh;
His urge to share Christ's crucifixion
Engulfed his soul in ecstasy.

Then, lo, there came the King from heaven
In garb of Seraphim arrayed.
His form, in angel wings enfolded,
A kindly countenance displayed;
But then the Cross that bore his members
His bitter sufferings portrayed.

The servant gazed upon his Savior,
Once suffering, now glorified;
The light and splendor of the Father,
But now so lowly, gentle, tried.
He understood the mystic message
To no mere human words allied.

The mountain peak burst forth in splendor,
As neighbors wondered down below;
The heart of Francis throbbed intensely
With flames of love for Christ aglow;