From Aleteia:
August 15 is not only the Solemnity of the Assumption of Mary: It’s also the start of St. Michael’s Lent, a practice that dates back to the 13th century and St. Francis of Assisi.
St. Bonaventure records in his biography (written between 1260-1266) how Francis, “was brought after many and varied toils unto a high mountain apart, that is called Mount Alverna. When, according unto his wont he began to keep a Lent there, fasting, in honor of St. Michael Archangel, he was filled unto overflowing, and as never before, with the sweetness of heavenly contemplation.” It was during this fast that St. Francis received the stigmata, the holy wounds of Jesus Christ.
Each year, St. Michael’s Lent begins on August 15 and ends on the feast of St. Michael the Archangel, September 29.