Mahagauri (Devnagari: महागौरी) is the eighth manifestation of goddess Durga and amongst the Navadurgas. Mahagauri is worshipped on the eighth day of Navratri. According to Hindu mythology, Goddess Mahagauri has the power to fulfill all the desires of her devotees.
Goddess Parvati made up her mind to perform an austere penance as advised by Sage Narad so that she might beget Lord Shiva as her husband. So, she renounced all the comforts of the palace and started performing penance in the forest. Her severe penance continued for many years. She braved heat and cold, rains and drought and terrible storms. Her body was covered with dust, earth, soil and leaves of trees. She had developed a blackish skin over her body. At last, Lord Shiva appeared before her and gave his solemn word that he would marry her. He bathed her vigorously by the holy waters of the Ganga emanating from his matted hair. The holy and sacred waters of Ganga washed off all the dirt clinging to Parvati’s person and she became white-complexioned and glorious , therefore Maa is compared with the conch, the moon and the white flower of Kunda. Radiant and compassionate, Maa Maugauri is usually depicted in a white or green saari and riding a bull. She is also known as Shwetambardhara.

Worship Maa Mahagauri to be free from clutches of material world and to remove sorrows from your life, for She will lead you to the path of virtue and inner power.

Mantra*Maa Mahagauri…

श्वेते वृषे समारूढा श्वेताम्बरधरा शुचिः |
महागौरी शुभं दद्यान्त्र महादेव प्रमोददा ||

Śvētē vr̥ṣē samārūḍhā śvētāmbaradharā śuciḥ |
mahāgaurī śubhaṁ dadyāntra mahādēva pramōdadā ||
श्वेते वृषे समारुढ़ा श्वेताम्बरधरा शुचिः
महागौरी शुभं दद्यान्महादेवप्रमोददा

ध्यान
वन्दे वांछित कामार्थेचन्द्रार्घकृतशेखराम्।
सिंहारूढाचतुर्भुजामहागौरीयशस्वीनीम्॥
पुणेन्दुनिभांगौरी सोमवक्रस्थिातांअष्टम दुर्गा त्रिनेत्रम।
वराभीतिकरांत्रिशूल ढमरूधरांमहागौरींभजेम्॥
पटाम्बरपरिधानामृदुहास्यानानालंकारभूषिताम्।
मंजीर, कार, केयूर, किंकिणिरत्न कुण्डल मण्डिताम्॥
प्रफुल्ल वदनांपल्लवाधरांकांत कपोलांचैवोक्यमोहनीम्।
कमनीयांलावण्यांमृणालांचंदन गन्ध लिप्ताम्॥

स्तोत्र
सर्वसंकट हंत्रीत्वंहिधन ऐश्वर्य प्रदायनीम्।
ज्ञानदाचतुर्वेदमयी,महागौरीप्रणमाम्यहम्॥
सुख शांति दात्री, धन धान्य प्रदायनीम्।
डमरूवाघप्रिया अघा महागौरीप्रणमाम्यहम्॥
त्रैलोक्यमंगलात्वंहितापत्रयप्रणमाम्यहम्।
वरदाचैतन्यमयीमहागौरीप्रणमाम्यहम्॥

कवच
ओंकार: पातुशीर्षोमां, हीं बीजंमां हृदयो।
क्लींबीजंसदापातुनभोगृहोचपादयो॥
ललाट कर्णो,हूं, बीजंपात महागौरीमां नेत्र घ्राणों।
कपोल चिबुकोफट् पातुस्वाहा मां सर्ववदनो॥

https://en.wikipedia.org/wiki/Mahagauri
http://ekikrat.in/Gauri-Puja
http://www.hindu-blog.com/2008/08/do-observe-perform-mangala-gowri-puja.html
http://www.drikpanchang.com/festivals/gowri-habba/swarna-gowri-vratha-date.html
http://www.indiaparenting.com/indian-culture/70_4065/significance-of-gauri-pujan.html
https://en.wikipedia.org/wiki/Gowri_Habba
http://www.rudraksha-ratna.com/mangal-gauri-puja-and-yajna_4907.php4
http://www.drikpanchang.com/navratri/ashwin-shardiya-navratri-dates.html