From Aleteia:
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
[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.