Shravana Mahatmya is described in Shri Skanda Purana. It is mentioned as conversation between Lord Shiva and the son of Lord Brahma named Sanatkumar. In other words, Sawan Mahatmya is described none other than Lord Shiva.
None of the day during Shravana Month is Shunya i.e. devoid of fasting. All Tithi during Sawan month are for fasting and perform austerity to please Lord of Destruction i.e. Lord Shiva. Weekdays fasting are also given due importance during this month.