Monday is dedicated to Lord Shiva. Shravan Somvar fast also has a scientific reason. Monsoon is at its peak and there is less sunlight, it makes digestive system sensitive and weak. The diet has to be as light and easy to digest as possible. Some people fast for four months of monsoon. Some take one meal during Shravan.
Devotees turns strict vegetarian. During this period, water-borne diseases spread rapidly. Fasts detoxify the system.
Whatever it is whether u are fasting for one day or one month it should be done properly, have lots of water, chass, lassi, milk, coconut water, fruits. Fasting foods like rajgira, sabudana, kuttu, samai are good. If u are eating one meal esp if it is dinner have it early.