Early records attest to St. Francis' reception of the stigmata around the feast of the Exaltation of the Cross.
While St. Francis is widely known for his love of animals and his radical poverty, he is also one of the earliest saints to have received the stigmata of Jesus Christ.
The stigmata is a miraculous reception of the wounds of Jesus on a saint’s body. It could be a single wound, or all five major wounds that Jesus suffered on the cross.
Shortly after his death, various biographers wrote about St. Francis of Assisi’s stigmata and how he bore Jesus’ wounds for a time before his death.
St. Bonaventure wrote about it in his Life of St. Francis, sometimes called the Legenda Major.
[O]n a certain morning about the Feast of the Exaltation of Holy Cross, while he was praying on the side of the mountain, he beheld a Seraph having six wings, flaming and resplendent, coming down from the heights of heaven…as the vision disappeared, it left in his heart a wondrous glow, but on his flesh also it imprinted a no less wondrous likeness of its tokens. For forthwith there began to appear in his hands and feet the marks of the nails, even as he had just beheld them in that Figure of the Crucified.
It is certainly providential that St. Francis received the wounds of Jesus on a feast day that commemorates the Holy Cross.
This miraculous event continues to be celebrated by the Franciscan Order as a special liturgical feast on September 17.
The reason why September 17 was chosen was because the Exaltation of the Cross is on September 14, and the closest available liturgical date at the time was September 17.
September 17 remains the only liturgical feast that honors the reception of the stigmata by a holy man or woman.