Mangalaa Gauree Vrat is kept on all Tuesdays falling in Shraavan Maas. This Vrat is for Suhaagin women (women with husbands).

Kathaa
In ancient times, there was a king named Chandraprabhu in Vimalaa Puree. Some Apsaraa advised him to ask his wife to do this Vrat, so he did ask his wife to do it. He asked his younger queen Vishaalaakshee to do this Vrat. She did not believe in this Vrat so she broke the thread of this Vrat and did not worship Mangalaa Gauree. As a result she became insane and started asking mango groves and Rishi "Where is Gauree?" At last by the grace of Gauree she became all right and started living happily and normally.

Another Kathaa
A similar story is given in "Kathaa for Simple Monday Fast"
One day during the exile, Dharmaraaya (Nand) asked Krishn - "O "Lord, kindly tell us which is that Vrat we have to undertake for Draupadee's Deergh Saumaangalya Bhaaghya / Yog while we are living in Aranya (forest). Krishn said - "O Nand Mahaaraaj listen - to protect the Stree's Paativratya, Maangalya and to experience the Sukh Santosh, there is one Vrat called - Mangalaa Gauree Vrat. In doing that you will all live happily, you will not have any Apa-Mrityu (accidental death) in this Ghor Aranya (jungle) and you will overcome all obstacles successfully and have good health. Previously once I was fighting against Raakshas, Lakshmee in Vaikunth in the form of Sarv Mangalaa, Shree Chakra Devtaa undertook this Mangalaa Gauree Vrat. Because of this Vrat, I could get more Shakti and could kill Raakshas for Lok Kalyaan. So Hey Nand, Go ahead, and do this Vrat in Shraavan Maas, Tuesday, pray that Maataa, give her your love and affection, your Draupadee will have Deergh Sumangal Bhaagya and asked Dharmaraaya to perform this Mangalaa Gauree Vrat through Dhaumya Rishi.

"O, Dharm Tanaya listen, previously there was a nation called Kundin Pur. There lived a business man called Dharmpaal who as per his name was very famous for his Dharm and was giving due importance to Braahman and poor people. Every day one Sanyaasee Bhikshuk used to come to the street of Dharmpaal but never came to the house of Dharmpaal and thus never accepted the Daan from him. Seeing this, Dharmapaal and his wife asked that Sanyaasee Bhikshuk - "Even though you come daily to our street for Bhikshaa, what makes you not to come and receive the Daan from us?" Sanyaasee said - "O couple, (Dharmapaal and his wife) You don't have children and hence I cannot receive the Bhikshaa from you." Hearing this, the couple forcibly put their Bhikshaa in the bag (Saddle) of that Sanyaasee . At this Sanyaasee got very angry and cursed the couple that - "Let there be no Santaan to you." and started to leave the place.

The couple immediately realized their mistake and pleaded guilty before that Sanyaasee. Sanyaasee then cooled down and said you both have to ride a black horse by tying a cloth covering your eyes to the Eastern part of this country. You will find a Shivaalaya (Shiv Mandir) where this horse will stop. You will see a Devee Paarvatee idol inside the temple, pray that Vigraha (idol) till you get her Darshan. Only then you will have Santaan and left the place after blessing the couple.

This couple started their journey on the same day. The horse went far away place inside a jungle and stopped near a pond. The couple got down from the horse and saw a Shiv Temple nearby. On seeing the Shiv temple, both got very happy and saw an idol of Paarvatee Jee also inside the temple. They started praying Paarvatee Jee. They prayed for many years in that jungle with full devotion and love. Paarvatee Jee pleased with the couple for their Bhakti and Shraddhaa, gave her Darshan to them and said - "I am pleased with your devotion and love, now you can ask for any Var (boon) you want." Hearing this Dharmpaal said - "Maataa, You are the protector of this Lok, we are worried of not having any Santaan, bless us the Santaan Bhaagya and free us from this Dosh." Devee immediately asked the couple - "Whether you want Vaidhavya-yut, Deergh Aayu (with widowhood, or long-lived) daughter or you want a short-lived blind son?" Hearing the couple could not control their tears and said - "Jaganmaataa, Instead of worrying and seeing day and night about the widow daughter, kindly bless us an Alpaayu (short lived) blind son." Jaganmaataa blessed the couple what they wanted and said - "Eat the fruit of that tree nearby and you will get what you want."

As per the Var of Devee, the couple ate the fruit of the tree standing nearby and went home. So when the time came, the wife of Dharmapaal gave birth to a boy. That baby was born blind. Even after this, the couple never grumbled. At last one day they performed his Sanskaar methodically and later performed all Sanskaar as the child grew and named that boy as Shiv. Years passed, when the boy was matured enough to marriage, wife of Dharmpaal requested her husband to search for a suitable bride for their son. Dharmpaal agreed to the request and said - "We will arrange our son's marriage once he returns from Kaashee Yaatraa." Accordingly he sent his son to Kaashee with his wife's brother and his father-in-law.

They reached Kaashee after visiting many Punya Kshetra. Here the boy Shiv, after taking bath in the Gangaa River visited Kaashee Vishwanaath Temple for Lord's Darshan and plead before God to grant him Aayush. Years passed, one day when he  went to Gangaa river for taking bath as usual, he saw many girls were chit-chatting there after taking bath and passing the time making fun at themselves sitting on the banks of river Gangaa. There was a girl by name Susheelaa who was making fun at one of her friends. Seeing this, her friend got wild and cursed Susheelaa - "One day you will become a Vidhavaa (widow)." Hearing this, Susheela replied - "You have cursed me, but I will not become Vidhavaa, nothing will happen to me, I will overcome your curse because of the effect of my Maa Mangal Gauree Vrat. Go ahead curse me again."

Shiv's uncle heard this conversation and started searching Susheelaa's address. One day, Shiv's uncle went to Susheelaa's parents and told them that he was interested in Susheelaa to marry his nephew Shiv. Susheelaa, her father and mother saw Shiv, all were pleased with his personality, intelligence and background and agreed for their daughter's marriage to him. Shiv had just completed his 16th year, and he got married to Susheelaa. On the same day, Susheelaa had the Darshan of Devee Maa in her dreams. Devee Maa said - "O my daughter Susheelaa, Your husband's Aayush is almost over. You will see a serpent today (Naagin), do not sleep, be awake, please that serpent by feeding her milk, and keep her closed in a Kalash (pot). Tomorrow is Tuesday, wrap the Kalash in a cloth and handover to your mother." and blessed Susheelaa.

Accordingly in that night she saw a serpent, she pleased the serpent with milk, as Maataa Gauree said to her. Serpent coolly went inside the Kalash. As soon as that serpent went inside the Kalash, Susheelaa wrapped that Kalash in a cloth with Bhakti and Shraddhaa gave it to her mother informing about the dream. Susheela's mother took the Kalash from her daughter and opened it. There was no serpent there but a diamond necklace was shining inside. She took out the diamond necklace and gave to her daughter and blessed her - Dheerg Sumangalee.

Here Shiv, cursing his fate, left for Haridwaar along with his uncle without informing Susheelaa or her parents. Shiv used to take bath in the Ganges River and prayed Pashupati Naath. Many Sanyaasee were doing their Tapas (penance) on the bank of Gangaa. Shiv took care of them, of their needs and able to get the blessings - "Deerghaayu" from them. Many days passed, here Susheelaa with the help of her father performed many Dharm Kaarya and prayed Maa and Shiv to give the eye sight to her husband Shiv. Seeing Susheelaa's Bhakti, Shiv got pleased, appeared before her and blessed Susheelaa.

So because of this Mangalaa Gauree Vrat, Shiv got the eye sight and Aayush. Shiv was so happy, he returned to the place where his wife Susheela was staying Susheelaa's father could not recognize Shiv and said - "You are not my daughters husband." Hearing this, Susheelaa came to Shiv and touched his feet and said to her father - "Father, He is my husband Shiv." After that, Shiv and Susheelaa and parents of both of them joined together to continue this Vrat for many years.

Krishn said to Dharmaraaya - "O Dharm Tanaya, Go and help Draupadee to perform this Vrat, so that you can live in this dense forest without any fear and made Dharmaraaya and Draupadee to undertake this Vrat. Listen Dharmaraaya - "This Vrat is very very important and sacred to all women. Whoever does this Vrat or help others to do this Vrat or read this story on Tuesdays during Shraavan Maas, Mangalaa Gauree will bless all of them with children, prosperity and happiness.

This is said in Hemaadri, Bhavishya Puraan with regard to Mangalaa Gauree Vrat. This Vrat is meant not only for women, but anybody can do this Vrat. If anybody keeps this fast after the marriage, the couple will not have any Maangalya Dosh, they will get Saubhaagya. If a woman gets Saubhaagya, the man will not have any obstacles in his married life. Therefore both Satee and Pati should do this Vrat. Even an unmarried girl can keep this fast to get a good husband.