Gauri Vratha is celebrated a day before Ganesh Chaturthi (Ganesh's creation day / birthday): Gauri created Ganesh, and understanding Gauri, perfecting Her manifestation, practicing her Yoga is important to any practices associated with Ganesh.
Consequently, it is a day traditionally set aside for devotions to be made to one's spouse, or for the desiring of a spouse to be known. For those who are married, it is typical to visit in-laws and be served special foods, so they can celebrate the marriage, and to attempt the conception of a child (another reason for this is that, if conceived in India, the child will be born when fresh fruits and vegetables are again available to eat, and disease-spreading insects are fewer).
Seeds are sprouted (typically large seeds, like corn) and the new shoots are meditated upon to help understand the "birth" of Gauri. A ritualized "goodbye" is said to Parvati: but to say goodbye means saying "hello" so Parvati is welcomed into the home, sometimes in the form of a long-married family member, friend or neighbor. This requires preparing the home as an offering to Parvati, with thorough cleaning, and decoration. But higher practice would have each individual bid goodbye to their previous manifestation of "Parvati" - "abandoning" the Jnana Yoga instructed by Parvati for a newer practice, the Yoga of Gauri, profound Bhakti Yoga. The evolution of practice is a natural consequence of its study. It was by the birth of Gauri that even greater understandings (Bodhi) was (literally) conceived of by Ganesh.
The story of Sati - Parvati - Kali - Gauri
Parvati loved Shiva dearly. She was the manifestation of Sati's devotion. Though Shiva and Parvati had a son and lived eternities happily together, trouble began after Parvati defended Shiva from His enemies in battle: He had been overcome, and out of love She rushed to Shiva’s defense.
But in Her war, by degrees, She became enraged, and quite insane. She began to wear the heads of Her slain enemies about her neck, a skirt made of severed arms, and covered herself in gore; lighting fire to everything, She herself was scorched. She had to become fearsome, that Her enemies would willingly submit to Her, and stop harming Shiva. She began to delight in destruction!
But even after all Her enemies were conquered, She was still so angry. She still delighted in destruction, and gore. She grew paranoid and violent; She saw enemies everywhere! She began to destroy everything! And even hurt Herself! Everyone was terrified of Her.
Because Shiva was the strongest, and Her husband, and She might still love him, and not hurt Him, even listen to Him, Shiva was asked by every living being to restrain Her. Yet Shiva knew She was stronger than Him; Parvati was Shakti. Only She could restrain Herself. But She was destroying everything She cared for, the entire world. So He had to do what could be done.
He approached her, and found She was ready to kill Him, as well - Shiva, Her true love, the one She had just saved! They were two halves of the same being, but in Her insanity, She attacked Shiva. It was a difficult fight: She was well armed, and a trained fighter. At first, He blocked all Her blows. Then, coming close to Her, He held Her in His arms, embracing her with love; in that embrace, Shiva tried to remind Her who He was, who She was, to remind her of love. He bent time and space to imprison Her, and tried to hold Her still for a moment... and for a moment, She would seem to collect Herself - yet the instant He loosened His grip, She would again attack Him. Again he would embrace her, and imprison her - and again, for a moment, she would regain self-control, only to lose it again.
In their battle, She struck Him and hurt Him in so many ways. They strove for what seemed an eternity; it was misery itself. She did not want to hurt Him, and He did not want to hurt Her; neither wanted to strive against the other - but She was not in control of Herself. She was in a rage. After some time Shiva had to accept the fact that war left Parvati utterly insane. Even today, you may know soldiers who do not come back from war the same way they left, or even wholly in their minds; or those whose troubled pasts or childhoods leave them insane and violent. Post-traumatic stress can destroy a person. She had been similarly injured by Her war. She felt no grief at the hurt she caused Shiva, or the world. Shiva was not insane, and so did feel grief. This grief weakened Shiva more and more. Eventually, She was so much stronger than Shiva! Shiva began to weep, and was no longer able to fight.
Her fury grew as Shiva’s weakness became evident: She was relying on Shiva to restrain Her; She had thought Him so strong! She admired His strength, and needed to believe He was invincible! Shiva persisted; but knew that He would be destroyed by Her before much longer. Everyone urged Shiva to kill Her in self-defense - but Shiva knew to do so would be to kill Himself. He was urged to kill Her in self-defense, in defense of the world, even if it destroyed Himself. They shared but one heart. This was the only hope for the world.
Shiva understood what He had to do - but could not bring himself to hurt his love, Parvati. He could not commit suicide, either (which would have destroyed Parvati, since they shared one heart). So, to save the world from Her destruction, Shiva laid down before Her, and permitted Her to kill Him - and thus bring about Her own destruction.
Shiva laid Himself down before Her altar in supreme sacrifice. She gladly accepted the gift. She raised Her sword for the victory blow, and then saw Shiva was weeping like a baby - for Her impending death, grieving for their love. And at that moment, She shared His grief. She threw down all Her weapons and wept with Shiva.
Shiva and Parvati held each other for a long time. She was scorched from battle, even Her hair was singed. Shiva held Her in both hands and smiled, then laughed. She asked what Shiva was laughing at? Shiva pointed at her blackened skin, and in the emotional moment could only say "Kali!" The word encompassed the "black" color of Her skin, the "grief" they both shared, the "one" heart they shared, the "strife" and "war" they had just shared, but it also carried the derogatory connotation of "imperfection," and "ugly."
She looked at herself and saw the filth that covered Her - not only the gore from Her war, but Shiva’s own blood and tears. She saw where She had hurt Herself. She did not laugh. She was horrified, and desired to purify Herself. She was non longer Parvati! She was Kali! Couldn't Shiva make her Parvati again? If Shiva could have given Her that grace, He would have: but no one can escape the consequences of their actions, good or bad. Shiva forgave Her, and urged Her to forgive Herself: She had been out of Her mind and not responsible for Her actions. Everyone - every being, even non-beings like She and Shiva - makes mistakes. But what She had done was necessary, and no mistake. The important thing was to recover, and do better. What was past could not be undone, but the future lay ahead. He loved Her as Sati, He loved Her as Parvati, He loved Her as Kali.
This did not comfort her; She was overcome by regret, and felt too weak to do better. So She wept. From these tears were borne the Thugs, and all Criminal Yogis, whose Dharma conflicts with their duty.
Shiva tried to comfort Her. She would not be comforted: She felt like she still had enemies to slay. Already, She felt the insanity growing again. Shiva urged her to forgive Her enemies. But She was becoming gripped by insanity again, and could not. She warned Shiva, She would attack Him again. She asked Him to feel no remorse if He hurt her. So She and Shiva began to struggle again, but now Shiva felt no grief. Shiva defended Himself with all His strength, understanding compassion would require Him to harm Kali. His great strength prevented Her aggression from hurting Him - or the world. And woke Her to Her own weakness. Suddenly, she regained self-control again.
When the fit had passed, She understood how the war had wounded Her. She understood at that moment She could no longer be Shiva’s Wife, or His friend, nor be near anything She loved; She could perform no duty. She had no reason to live; She would hurt anything near Her or dear to Her. She vowed She would destroy Herself upon Shiva’s altar, for She had become His enemy, Her own enemy. Death was better than that incurable injury which caused so much distress to Her and those who cared for Her.
Remembering when She had burned herself upon His altar in the form of Sati, Shiva held her close and wept. And as we wept a river of tears, all Her Black was washed away - Kali was washed away! The Black took a hollow form and, possessed of insanity and wrath, tried to hurt Shiva, but was powerless. The hollow Kali then tried to destroy the world, but was powerless. The hollow Kali then fled across every world, in impotent anger and fury. This hollow Kali was Kausiki: hollowness and without form, pure power, without mind, heart or body, a pure selfish malice, devoid of love. No longer His wife, Kausiki left Shiva’s presence. He was glad when Kausiki left.
When Kausiki left, what was left of Kali was Gauri: the body, heart and mind underneath that shell of blackness. All that Black shell had been washed away by tears, and utterly purified! This was pure Shakti, and She was radiant! Kali was no more.
Gauri stood before Shiva; her skin as white as if She had never seen the sun in all Her life. Indeed, She had been there all along, under that Black skin. Behind all that blackness. His wife had forgotten who She truly was: not Kali, not Parvati, not even Sati. But Shiva remembered when he wept. And this grief manifested His understanding of Shakti; Shakti had taken form before Shiva, as Gauri, by this devotion. Now, Shakti was embodied in a more perfected form, as Gauri. Gauri was everything Shakti wanted to become. She embraced Shiva, and at that moment taught Shiva to let go of all the pain He still held in His heart for Sati, Parvati and Kali; when His profound distress from sympathy in Kali's suffering was extinguished, He learned better wisdom.
The story of the Marriage of Gauri
Gauri made herself worthy of Shiva's love through devotional Yoga. For 16 years, she practiced perfectly. Shiva noticed Gauri, and asked Gauri what it was that she desired from his love. She said that she desired nothing less than to be Shiva's Ardhangini, and that Shiva would have no other wife but her. She wanted Shiva to love her as she loved Him, constantly, purely, and solely. She made profound vows. Shiva was touched, and agreed, "I shall have not even the thought of another in my heart but you." Shiva laid his heart bare for Gauri to see, and it was like a mirror: she saw only her own image in it. Gauri was so happy, she knelt down before Shiva, and crying with joy, gave thanks. But Shiva lifted her up, and held her tightly: she was His Ardhangini, half of his being - and should not bow before Him. They rejoiced together for a very long time. "Eternities came and went."
One day, Shiva sat on a rock in Kailasa, and Gauri was sitting beside Him; she was so happy because she was with Him. And then, all of a sudden, everything seemed wrong to Gauri. She actually began to tremble with fear, doubt, anguish. All her strength left her, she grew limp and felt weakness like a poison pushed through her veins by her heart. She saw in Shiva's heart not her own reflection, but the image of another! A beautiful goddess! Oh, Gauri was so jealous! This goddess was beautiful, more beautiful than she ever could be! Young, desirable - and in the same joyous love Gauri had known. This woman was beloved of Shiva! Jealousy became hatred. And because both Shiva and Gauri shared one heart, the poison passed through Shiva's body, and He began to suffer as well.
Beyond time, beyond space, the two sank into misery. Gauri began to doubt that she kept her vows, for why else would Shiva have broken His? She knew though that she had kept every promise. She faded from existence, and Shiva lost his own breath; the two were one, inseparably one. Like fish need water, Shiva could not survive. Shiva sought in every world. Eventually, on a mountaintop, in a world of darkness, the ghost of Gauri sat - no happiness existed in her; she was consumed by desire, aversion, hatred. She had become a spirit.
Shiva took her in His arms, so glad to have found her. He pressed her close to His heart, and she began to breathe again. He asked her, "what had happened? Why had she become so tormented?" He kissed her forehead, her cheeks, her lips, her head; he called out her name over and over again. Gauri! Gauri! But she could not respond, she was consumed by despair. And so Shiva wept. And as his heart began to break, Gauri's own heart felt pain. The pain woke her a little, her spirit form was in agony. She held Shiva closer, and felt life; she began to feel terror - at losing Shiva. Then she remembered that other woman in Shiva's heart - and violently pushed Shiva away from her and in an angry tirade explained how Shiva had been so cruel to her, how He had made her existence meaningless and destroyed her. And she ripped open His chest to lay bare His heart.
But it was her face she saw in the heart. Both she and Shiva now wept. She asked what happened to the other woman who was in His heart, but now was not there? The woman who had taken her place in Shiva's heart?
Shiva stopped weeping a moment. Then, understanding what had happened, smiled. Then He laughed. "Her?" He laughed louder and louder, increasingly relieved - it seemed so cruel to Gauri. The beautiful woman's face was again in Shiva's heart. "Look - it is Parvati. It is your own reflection!" Shiva laughed.
Gauri felt confused, then understood. Then she was embarrassed a moment. But then she too, smiled. In an instant she and Shiva were sitting in Kailasa again, the sun was shining just as before - there was no regret: beyond time, beyond space, it did not matter how long the two had been in misery - they were now happy again. As if they had always been happy. And she saw her beautiful smile in the reflection. In Shiva's heart. And laughed with relief and happiness.
Shiva then made a promise to Gauri, that this would never happen again. "From now on, you and I shall not be Ardhanginis, two bodies with a single heart, but a single united body." They embraced, and in that hug held each other so tightly that the two became one. By devotional Bhakti, they became the half-male, half-female Ardhanarishwara.