In the Hindu calendar seasons follow the sun, months follow the moon and days, both the sun and the moon. Since the lunar month is 29 days 12 hours 44 minutes, and 3 seconds long, the lunar year comprises 354 days, 8 hours, 48 minutes, and 36 seconds compared to 365 days, 5 hours, 48 minutes, and 46 seconds of the solar year.
Therefore, 30 solar months (two and a half years) equal 31 lunar months i.e. the gap increases to a month every two and a half years. For this reason, the Hindu lunar calendar has an extra month every two and a half years. This extra month is called Adhik Mas.
The month of Adhik is considered to be more auspicious than other months, and is referred to as the month of God. Those observing extra penance (Bhakti) and benevolence during this month gain extra blessings from God and their sins are washed away. During this month, auspicious events such as weddings are avoided.
Each solar year is divided into 12 lunar months. Each lunar month is further divided into 2 equal halves. The first half (Sud) is referred to as the bright half and this corresponds to the 15 days from the new moon to when a full moon is visible (Punam). The dark half (Vad) refers to when the moon is waning, i.e. the 15 days from the full moon to the new moon (Amas).
Lunar days in the Hindu calendar are called Tithis. They are calculated using the difference of the longitudinal angle between the position of the sun and moon. Because of this, Tithis may vary in length. Consequently, the Tithi may or may not have changed by sunrise. That is why we find, at certain times, a Tithi being omitted and, at certain times, two consecutive days sharing the same Tithi.
The reference point, which is used as the starting point of the most widely used calendar, is the coronation year of King Vikram. It is for this reason that Hindu dates use 'Samvat' prior to the year or are prefixed with V.S. to define Vikram Samvat. The Vikram Era, or the Vikram Samvat, began in 57 BCE. To calculate the corresponding year of the Common Era, 57 years should be subtracted from the Hindu year if the date falls between the beginning of the Hindu year and the end of the Western year i.e. between Kartak Sud 1 and 31st December. If the date falls between the beginning of the Western year and the end of the Hindu year i.e. between 1st January and Aaso Vad 30, then only 56 years should be subtracted.
The Hindu months and the Gregorian (solar system based) calendar months are as follows:
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