The Tamil calendar (தமிழ் நாட்காட்டி) is a sidereal solar calendar used by the Tamil people of the Indian subcontinent.^[1][2] It is also used in Puducherry, and by the Tamil population in Sri Lanka, Malaysia, Singapore, Myanmar and Mauritius.

It is used in contemporary times for cultural, religious and agricultural events,^[3] 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,India .^[4]

The calendar follows a 60-year cycle that is also very ancient and is observed by most traditional calendars of India and China. This is related to 5 12-year revolutions of Jupiter around the Sun and one that adds up to 60 years and the orbit of Nakshatras (stars) as described in the Surya Siddhanta.

In the Gregorian year 2024, the Tamil year starts on 14 April 2024, Kaliyuga 5126. The Vikrama and Shalivahana (Saka) eras are also used.

There are several references in early Tamil literature to the new year. Nakkeerar, Sangam period author of the Neṭunalvāṭai, wrote in the third century CE that the Sun travels each year from Mesha/Chittirai in mid-April through 11 successive signs of the zodiac.^[5] Kūdalūr Kiḻar in the third century CE refers to Mesha Rāsi/Chittirai i.e. mid-April as the commencement of the year in the Puṟanāṉūṟu.^[6][7] The Tolkappiyam is the oldest surviving Tamil grammar text that divides the year into six seasons where Chihthirrai i.e. mid-April marks the start of the Ilavenil season or Summer.^[8] The 5th century Silappadhigaaram mentions the 12 rāsigal or zodiac signs that correspond to the Tamil months starting with Mesha/Chittirai in mid-April.^[9] The Manimekalai alludes to this very same Hindu solar calendar as we know it today^[10] Adiyarkunalaar, an early medieval commentator or Urai-asiriyar mentions the twelve months of the Tamil calendar with particular reference to Chittirai 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.^[11]

The Tamil New Year follows the nirayanam vernal equinox^{[12][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.^[13] This also coincides with the traditional new year in Burma, Cambodia, Laos, Sri Lanka, Bangladesh, Nepal, and Thailand.

The days of week (Kiḻamai) in the Tamil calendar relate to the celestial bodies in the solar system: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn, in that order. The week starts with Sunday.

The number of days in a month varies between 29 and 32. These are the months of the Tamil Calendar.

The Sanskrit month starts a few weeks ahead of the Tamil month, since the Tamil calendar is a solar calendar, while the Sanskrit calendar is a lunisolar calendar.^[14]

The Tamil year, in keeping with the old Indic calendar, is divided into six seasons, each of which lasts two months:

The 60-year cycle is common to both North and South Indian traditional calendars, with the same name and sequence of years. Its earliest reference is to be found in Surya Siddhanta, which Varahamihirar (550 CE) believed to be the most accurate of the then current theories of astronomy. However, in the Surya Siddhantic list, the first year was Vijaya and not Prabhava as currently used. There are some parallels in this sexagenary cycle with the Chinese calendar.^[15][16][17] The Surya Siddhanta and other Indian classical texts on astronomy had some influence on the Chinese calendar^[18] although it merits attention that the sexagenary cycle in China is itself very old.

After the completion of sixty years, the calendar starts with the first year. This corresponds to the Hindu "century." The Vakya or Tirukannitha Panchangam (the traditional Tamil almanac) outlines this sequence. It is related to the position of the planets in the sky with respect to Earth. It means that the two major planets Sani/Saturn (which takes 30 years to complete one cycle round the Sun) and the Viyaḻan/Jupiter (which takes 12 years to complete one cycle round the Sun) comes to the same position after 60 years.

The following list presents the current 60-year cycle of the Tamil calendar:^[19]