Upvaas, or fasting, in hinduism is mainly observed for the fulfillment of a vow. Upvas is also referred as 'Vrats.' But 'Vrat' has a wider meaning and it is not just curtailed to the fasting observed for material or other gains. Each day in a week is dedicated to one or more of the gods in the hindu pantheon. A particular deity is appeased by the devotee on a particular day. Thus monday is usually dedicated to Lord Shiva.
Upvass, or fasting, on Mondays begins at sunrise and ends at sunset. On the day, food is only eaten after evening prayer. Lord shiva and Goddess Parvati are worshipped on the day. Of course, no worship begins without remembering Lord Ganesha. The fasting on Mondays in the shravan month is considered even more auspicious.
It is believed that those people who observe Upvaas on monday will have their desires fulfilled and will be blessed with wisdom. In some areas, fasting on Mondays is observed by unmarried women to get an ideal husband. The reason for fasting on a day is usually traced back to a particular incident in one of the hindu Puranas or folk tradition. There are numerous. myths associated with a particular fast. The stories vary from region to region and from community to community.