The Tamil calendar is a sidereal Hindu calendar used by the Tamil people of the Indian subcontinent. It is also used in Puducherry, and by the Tamil population in Malaysia, Singapore, Mauritius and Sri Lanka. Tamil Nadu farmers greatly refer to this. It is used today for cultural, religious and agricultural events, with the Gregorian calendar largely used for official purposes both within and outside India. The Tamil calendar is based on the classical Hindu solar calendar also used in Assam, West Bengal, Kerala, Manipur, Nepal, Odisha, Rajasthan and Punjab This shows Hindus were well versed about planetary movements.
There are several festivals based on the Tamil calendar. The Tamil New Year follows the nirayanam vernal equinox[page needed] and generally falls on 14 April of the Gregorian year. 14 April marks the first day of the traditional Tamil calendar and is a public holiday in the state of Tamil Nadu, Sri Lanka and Mauritius. Tropical vernal equinox fall around 22 March, and by adding 23 degrees of trepidation (oscillation) to it, we get the Hindu sidereal or Nirayana Mesha Sankranti (Sun's transition into nirayana Aries). Hence, the Tamil calendar begins on the same date in April which is observed by most traditional calendars of the rest of India – Assam, Bengal, Kerala, Odisha, Manipur, Punjab etc. This also coincides with the traditional new year in Burma, Cambodia, Laos, Sri Lanka, Bangladesh, Nepal and Thailand.
The calendar follows a 60-year cycle which is also very ancient and is observed by most traditional calendars of India and China. According to popular belief it is related to 5 revolutions of Jupiter around the Sun, and also to 60-year orbit of Nakshatras (stars) as mentioned in Surya Siddhanta.
There are several references in early Tamil literature to the new year. Nakkirar, Sangam period author of the Neṭunalvāṭai, wrote in the third century CE that the Sun travels each year from Mesha/Chitterai in mid-April through 11 successive signs of the zodiac. Kūdalūr Kizhaar in the third century CE refers to Mesha Raasi/Chitterai i.e. mid-April as the commencement of the year in the Puṟanāṉūṟu. The Tolkaapiyam is the oldest surviving Tamil grammar text that divides the year into six seasons where Chitterai i.e. mid-April marks the start of the Ilavenil season or Summer. The 5th century Silappadikaaram mentions the 12 Raasis or zodiac signs that correspond to the Tamil months starting with Mesha/Chitterai in mid-April. The Manimekalai alludes to this very same Hindu solar calendar as we know it today Adiyarkunalaar, an early medieval commentator or Urai-asiriyar mentions the twelve months of the Tamil calendar with particular reference to Chitterai i.e. mid-April. There were subsequent inscriptional references in Pagan, Burma dated to the 11th century CE and in Sukhothai, Thailand dated to the 14th century CE to South Indian, often Vaishnavite, courtiers who were tasked with defining the traditional calendar that began in mid-April.