The story behind this feast is beautifully told here.
General information about the stigmata can be found here.
And Friar Charles has a beautiful reflection on this feast.
In his Letter to All the Faithful, Francis wrote:
Because he suffered so much for us and gave us such gifts and will continue to bestow them in time to come, let every creature that is in heaven and on earth and in the sea and in the depths of the abyss render praise to God, and glory and honor and blessing, because he is our virtue and our strength: he who alone is good, who alone is Most High, alone almighty, wondrous, glorious; who alone is holy, worthy of praise, blessed forever and ever. Amen.