In Hinduism the term Vrat has been widely known since the early Vedic period. In Rigveda the word Vrata occurs just over two hundred times alone. Vratas throughout Hindu tradition is contemplated as part of dharma for each individual placing each of the gods to their highest level. In Sanskrit more universally, vrata denotes a "religious vow". Vrata signifies a set of rules and discipline stemming from the verbal root 'vrn' which literally means "to choose". These vows are said to be imperative ritualistic obligations serving on the sacrificer for several reasons.
The rationales behind and the kind of Vrata vary depending on the caste, region etc. One Gods Vrata may be different from another, Vrata most commonly involve certain rituals such as fasting, worship, kathas etc. Vrata are closely associated to Bhakti devotional rituals, most Vrata are performed to gain moksha, to recuperate life, to alleviate past karmas and most prominently to please the gods n goddesses.
It's said that the Shravan Somvar Vrat/fast was observed first by Goddess Parvati in order to get Lord Shiva as her husband. By the auspiciousness of this fast she got Lord Shiva in form of her husband. So the Monday fast is known to be kept by girls for getting their wish fulfilled of their choice husband.
The 16 Monday fasting (Solah Somwar) is considered the most favourable out of all the fast of Lord Shiva. This fast is kept by unmarried girls for a happy and peaceful life. The fast of 16 Mondays is kept by married girls for the longevity of their husbands. Monday fast is kept by men for getting child, wealth and prestige. Monday Vrat should be observed by those people who have moon in weak position in their Kundali, or moon is not able to give favorable results. Also, to remove pessimism and increase mental happiness, this fast is observed. Lord of Moon is Shiva, Because he holds the moon on his head. Out of all the planets, moon is the closest to earth, Hence, it effects our life and mind. So, the people whose birth sign, birth Nakshatra is of moon, they must observe the fast of Monday. This Vrat is also observed for health of mother and maternal happiness ! - 56 minutes ago