Shravana Mahatmya is performed to seek blessing of Lord Shiva.
All Mondays or Somwar which fall during Shravan month are considered highly auspicious for fasting to seek blessing of Lord Shiva and known as Shravan Somwar. Most states follow regional calendars due to which dates of Sawan Somwar might differ by 2 weeks.
In North India, Shivaratri which falls during the auspicious month of Shravana is known as Sawan Shivaratri. Except Hindi belt in North India, in rest of India Sawan begins one or two days after Sawan Shivaratri.
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.