Nanda Vrata (or Nandyāvrata) was once observed by Devi Sati for good husband (Lord Shiva). This fast continues through one full year. It is described in Shiva Mahapurana, Rudra Samhita (Sati Khanda), Chapter 15 as below.
अथ सापि स्वयं भर्ग प्राप्तमैच्छतदान्वहम्।
पितुर्मनोगर्ति ज्ञात्वा मातुर्निकटमागता।११।।
Sati on the other hand also got anxious to achieve Siva. Realising the mental condition of her father, she went to her mother.
पप्रच्छाज्ञां तपोहेतोः शङ्करस्य विशालधीः।
मातुः शिवाय वीरिण्याः सा सती परमेश्वरी॥१२॥
She said to her mother, "I intend to perform tapas in order to achieve Siva as my husband, I seek your permission.” These were the words spoken by Sati Paramesvari to her mother.
तत: सती महेशानं पर्ति प्राप्मुं दृढ़व्रता।
सा तमाराधयामास गृहे मातुरनुज्ञया।।॥१३॥
Then getting consent of her mother, Sati, with a firm mind, started adoring Siva in her home in order to receive him as her husband.
अाश्विने मासि नन्दायां तिथावानर्च भक्तितः।
गुडौदनैः सलवणैर्हरं नत्वा निनाय तम्।। १४॥
In the month of Asvin, (September-October), on the Nanda-tithi (i.e. first, sixth and the eleventh day of the lunar) fortnight, she adored Siva with great devotion offering
the cooked rice with jaggery, and salt, for a month.
कार्तिकस्य चतुर्दश्यामपूपैः पायसैरपि।
समाकीर्णैः समाराध्य सस्मार परमेश्वरम्।।१५।।
On the fourteenth day of the month of Kārtika, she adored and meditated upon Siva offering the Sweet pies and pudding.
मार्गशीर्षेऽसिताष्टम्यां सतिलैः सयवौदनैः॥
पूजयित्वा हरं काली निनाय दिवसान् सती।।१६।।
On the eighth day of Marga-Sirsa (November-December) Satī adored Siva with cooked barley and sesamum seeds, spending the days in his devotion.
पौषे तु शुक्लसतम्यां कृत्वा जागरण निशि।
अपूजयच्छिवं प्रातः कृशरान्नेन सा सती॥१७॥
On the seventh day of the bright half of the month of Pausa (December-January) Sati remained awake during the night and adored
Siva in the morning with cooked rice and Krisara (khicadi).
माघे तु पौर्णमास्यां सा कृत्वा जागरणं निशि।
On the full-moon night of Magha, (January-February) she remained awake during the night, and adored Siva on the banks of the river wearing wet clothes.
तपस्यासितभूतायां कृत्वा जागरणं निशि।
विशेषतः समानर्च शैलूषैः सर्वयामसु।।१९।।
She, on the fourteenth day of the dark half of the month of Phalguna (February-March) remained awake during the night and performed special worship of Siva, with wood
apple fruits and leaves in a span of every three hours.
चैत्रे शुक्लचतुर्दश्यां पलाशैर्दमनैः शिवम्।
अपूजयद्दिवारात्रौ संस्मरन् सा निनाय तम्॥२०।।
On the fourteenth day of the bright half of the month of Caitra (March-April), Siva was worshipped by her with palasa (Butea frondosa) and damana flowers offered during
the day and the night. She spent the rest of the month reciting his name.
पूजयित्वा सती रुद्रं गव्यैर्मासं निनाय तम्॥२१॥
After adoring him with cooked rice and sesamum seeds, on the third day of the bright half of the month of Magha (January February) she consumed throughout the rest of the month the products of the cow's milk.
ज्येष्ठस्य पूर्णिमायां वै रात्रौ सम्पूज्य शङ्करम्।
वसनैर्वृहतीपुष्पैर्निराहारा निनाय तम्।।२२।।
Then she adored the lord Siva with the offering of cloths and Brhati flowers on the full moon night of the month of Jyestha (May-June) she observed the fast throughout the month.
आषाढस्य चतुर्दश्यां शुक्लायां कृष्णवाससा।
बृहतीकुसुमैः पूजा रुद्रस्याकारि वै तथा।२३।
On the fourteenth day of the bright fortnight of the month of Asadha (June-July), she clad herself in a black cloth, and adored Rudra, offering Brhati flowers.
श्रावणस्य सिताष्टम्यां चतुर्दश्यां च सा शिवम्।
On the eighth and the fourteenth days of the month of (July-August) Sravana, she
worshipped Siva, offering the yajnopavits and the clothes.
भाद्रे कृष्णत्रयोदश्यां पुष्पैर्नानाविधै: फलै:।
सम्पूज्य च चतुर्दश्यां चकार जलभोजनम्।।२५।।
On the thirteenth day of the dark half of Bhādrapada, (August-September) she
worshipped Siva, offering various fruits and flowers She took water on the fourteenth day.
नानाविधैः फलैः पुष्पैः सस्यैस्तत्कालसम्भवैः।
चक्रे सुनियताहारा जपन्मासे शिवार्चनम्।।२६॥
She kept strict control over her food and reciting various mantras she worshipped Siva, offering different fruits, flowers and leaves which were quite fresh and readily available.
सर्वमासे सर्वदिने शिवार्चनरता सती।
The goddess took to the human form at will and was always engaged in the worship of Siva for days and months remaining firm in her resolve.
इत्थ नन्दाव्रतं कृत्स्त्र समाप्य सुसमाहिता।
दध्यौ शिवं सती प्रेम्णा निश्चलाऽभूदनन्यधीः।।२८।।
Thus she completed the fast of Nandāvrata concentrating her mind to Siva,
she was quite firm in her resolve and did not think of any one else.
Please note that, In my opinion it should be
वैशाखशुक्लतृतीयायां instead of
माघशुक्लतृतीयायां in 21st verse.