Vikram Samvat (Nepali: विक्रम सम्वत्) (abbreviated as V.S., VS, B.S. or BS) Nepali: नेपाली पात्रो; Listen ) is an era used in India and Nepal, just like the Christian era started in 1 CE. The Vikram Samvat started in 58/57 BCE in southern (purnimanta) and 57/56 BCE in northern (amanta) systems of Hindu calendar. The era is named after king Vikramaditya.
The Vikram Samvat calendar is 56.7 years ahead (in count) of the solar Gregorian calendar. For example, the year 2073 BS began in 2016 CE and will end in 2017 CE. The new year begins with the first day of month Baishakh, which usually falls in April–May in the Gregorian calendar. The first day of the new year is passionately celebrated in a historical carnival that takes place every year in Bhaktapur, called Bisket Jatra.
The Rana rulers of Nepal made it their official calendar. In India, the reformulated Saka Calendar is officially used, although in the Hindi version of the Preamble of the Constitution of India, the date of adoption of the constitution, 26 November 1949, is presented in Vikram Samvat (Margsheersh Shukla Saptami Samvat 2006). There have been calls for the Vikram Samvat to replace Saka as India's official calendar.
The classical Vikram Samvat uses lunar months and solar sidereal years. Because 12 months do not match a sidereal year exactly, correctional months are added (adhika-masa see Hindu_calendar#Extra months (Adhika Māsa)) or occasionally subtracted (kshaya masa).A Tithi or lunar day is defined as the time it takes for the longitudinal angle between the moon and the Sun to increase by 12°. Tithis begin at varying times of day and vary in duration from approximately 19 to approximately 26 hours.A Paksa (also Pakṣa) or lunar fortnight consists of 15 tithis.A Māsa or lunar month (approximately 29.5 days) is divided into 2 Pakṣas: the one between new moon and full moon (waxing) is called gaura or (bright) or Śukla Pakṣa; the one between full moon and new moon (waning) Kṛiṣhṇa (dark) pakshaA Ṛitu (or season) is 2 MāsaAn Ayana is 3 ṚitusA year is two Ayanas 
Months of the Nepali Bikram Samvat:
The exact length of each month is the time taken by the Sun to move through a full zodiac sign.
According to popular tradition, the legendary king Vikramaditya of Ujjain established the Vikrama Samvat era after defeating the Śakas.
Kalakacharya Kathanaka ("An account of the monk Kathanaka") by the Jain sage Mahesarasuri gives the following account: Gandharvasena, the then-powerful king of Ujjain, abducted a nun called Sarasvati, who was the sister of the monk(citation missing). The enraged monk sought the help of the Śaka ruler King Sahi in Sistan. Despite heavy odds but aided by miracles, the Śaka king defeated Gandharvasena and made him a captive. Sarasvati was repatriated, although Gandharvasena himself was forgiven. The defeated king retired to the forest, where he was killed by a tiger. His son, Vikramaditya, being brought up in the forest, had to rule from Pratishthana (modern Paithan in Maharashtra). Later on, Vikramaditya invaded Ujjain and drove away the Śakas. To commemorate this event, he started a new era called the "Vikrama era". The Ujjain calendar started around 56-58 BCE, and the subsequent Shaka era calendar was started in 78 CE at Pratishthana.
The association of the era beginning in 57 BCE with Vikramaditya is not found in any source before the 9th century CE. The earlier sources call this era by various names, including Kṛṭa (343 CE and 371 CE), Kritaa (404 CE), the era of the Malava tribe (424 CE), or simply, Samvat.
The earliest known inscription that calls the era "Vikrama" is from 842 CE. This inscription of Chauhana ruler Chandamahasena was found at Dholpur, and is dated Vikrama Samvat 898, Vaishakha Shukla 2, Chanda (16 April 842 CE). The earliest known inscription that associates this era with a king called Vikramaditya is dated 971 CE. The earliest literary work that connects the era to Vikramaditya is Subhashita-Ratna-Sandoha (993-994 CE) by the Jain author Amitagati.
For this reason, multiple authors believe that the Vikram Samvat was not started by Vikramaditya, who might be a purely legendary king or the title adopted by a later king who renamed the era after himself. V. A. Smith and D. R. Bhandarkar believed that Chandragupta II adopted the title Vikramaditya, and changed the name of the era to "Vikrama Samvat". According to Rudolf Hoernlé, the king responsible for this change was Yashodharman: Hoernlé also believed that he conquered Kashmir, and is same person as the "Harsha Vikramaditya" mentioned in Kalhana's Rajatarangini.
Earlier, some scholars believed that the Vikrama Samavat corresponded to the Azes era of the Indo-Scythian (Śaka) king King Azes. However, this was disputed by Robert Bracey following discovery of an inscription of Vijayamitra, which is dated in two eras. The theory seems to be now thoroughly discredited by Falk and Bennett, who place the inception of the Azes era in 47/6 BCE.
After the rise of the Rana oligarchs in Nepal, Vikram Samvat came into unofficial use along with the official Śaka era for quite some time. They discontinued the Śaka era in its 1823rd year and replaced it with the Vikram Samvat for official use since then. Vikram Samvat came into official use in its 1958th year.
The traditional New Year of Vikram Samvat is one of the many festivals of Nepal, marked by parties, family gatherings, the exchange of good wishes, and participation in rituals to ensure good fortune in the coming year. It occurs in mid-April each year, and coincides with the traditional new year in Assam, Bengal, Burma, Cambodia, Kerala, Kashmir, Manipur, Orissa, Punjab, Sri Lanka, Tamil Nadu and Thailand.
In addition to Nepal, the Vikram Samvat calendar is also recognized in North and East India, and in Gujarat among Hindus. In Buddhist communities, the month of Baishakh is associated with Vesak or Buddha's Birthday. It commemorates the birth, Enlightenment and passing of Gautama Buddha on the first full moon day in May, except in a leap year when the festival is held in June. Although this festival is not held on the same day as Pahela Baishakh, the holidays typically fall in the same month (Baishakh) of the Bengali, Hindu, and Theravada Buddhist calendars, and are related historically through the spread of Hinduism and Buddhism in South Asia.
In Gujarat, the second day of Diwali is celebrated as the first day of the Vikram Samvat calendar which is the first day of the month Kartik.