The Indian calender is based on both the sun and the moon; it uses a solar year but divides it into 12 lunar months. A lunar month is precisely 29 days 12 hours 44 minutes and 3 seconds long. Twelve such months constitute a lunar year of 354 days 8 hours 48 minutes and 36 seconds. The Western calendar is based on the sun, in which a year is the time required for the earth to complete one orbit around the sun. This precisely measures 365 days 5 hours 48 minutes and 46 seconds. To help the lunar months coincide with the solar year, the practice of inserting an intercalary (extra) month arose. So 60 solar months = 62 lunar months. Hence an extra month, called the Adhik Mas, is inserted every 30 months i.e. every 2 ½ years.
The Solar Calendar. This begins with Vernal Equinox, when the Sun comes in Aries Sign. The 1st month of lunar calendar, Chaitra, corresponds to the Solar month of Aries. The Aries Month begins on or close to 12th day of April, and this day is celebrated as New Year Day known as Vaishakhi.
Makar Sankranti, which corresponds to the entry of the Sun into Makar Rasi, the sign of Capricorn, occurs on January 11- 12th every year. Grishma Ritu, commencing at the start of Mithun Rasi (Gemini in mid June) is the "hot summer." The Raining season, Varsha Ritu, begins in Simha Rasi (Leo mid August). Sarad Ritu, the season of fruits, begins in Thula Rasi (Libra mid October). Hemant Ritu, the cold season begins in mid December. Sishir Ritu, the last season of the year, begins in the Kumbha Rasi (Aquarius mid February)
Lunar days in the Indian 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 the time the day has changed at sunrise. And that is why we find at certain times a tithi being omitted, and at certain times, two consecutive days sharing the same tithi.
In the Indian calendar, seasons follow the sun; months follow the moon; and days, both the sun and the moon. The era in the Indian calendar is called the Vikram Era, or the Vikram Samvat, which began in 57 BCE. To calculate the corresponding year of the Common Era, 57 years should be subtracted from the Indian year if the date falls between the beginning of the Indian year and the end of the Western year i.e. between Kartak sud 1 and 31 December. If the date falls between the beginning of the Western year and the end of the Indian year i.e. between 1 January and Aso vad 30, then only 56 years should be subtracted.