Monday fasts starting in Shravan month are observed for Lord Shiva. It is especially done by unmarried girls with an idea to get a perfect husband. There are many interpretations to the same.
One of the interpretation and thinking is as follows:
Shravan is the first month of Chaturmas and the Monday fast starts in Shravan and ends after 16 weeks (16 Monday)s which means fast is done throughout the four months of Chaturmas.
Chaturmas is of the first four months of Dakshinayana period with longer nights and shorter days. It is a period of natural negativity of the mind with more suicide and abnormal mental behaviours.
Two Hindi Film songs summarize the Chaturmas, .Sawan ka mahina pawan kare shore indicates that in the month of Shravan, there is Andhi outside, the same is also experienced mentally by susceptible individuals. That means these people have unstable mind during the month of Shravan.
The second song is Mere naina sawan bhadon phir bhi mera man payasa, which means that desires increase in the month of shravan and remain unfulfilled in bhado.
Unmarried girls during Chaturmas are, therefore, likely to face negative state of mind and may get attracted to other men and end up with pre-marital affairs. Premarital affairs may be devastating for the girl as she may not get perfect husband later on. The 16 Mondays of Chaturmas observed in the form of Shiva Puja Vrat restrain an unmarried girl throughout the Chaturmas period from getting diverted to the boy community.