Significance

The Hindus had developed a system of calendrics that encapsulates vast periods of time. For computing the age of the earth and various geological and other epochs, as well as the age of mankind, they still employ a Tamil calendar derived from ancient astronomical data, known as the Tirukkanida Panchanga (cf. The Secret Doctrine, 2:49-51).
This calendar contains a calculation of something over three hundred million years for the age of the present earth since sedimentation occurred, and a period of somewhat more than eighteen million years since the first appearance of our mankind.
The 10th Tamil month, called Thai, falls in the mid-January every year. It is celebrated with much enthusiasm among the Tamil Community all over the world. It is marked by gifting new dresses for the family members and giving prayers to God wishing for prosperity among the people for the coming year. The month of Thai and the fifth month Aavani are considered very auspicious and most marriages usually happen during those months than the other months of the year.
The fourth month Aadi is considered inauspicious, so usually weddings do not happen in the month. Aadi is also the month of preparation for next crop cycle by farmers. Hence, farming communities avoid major events like weddings in this month. On the contrary (or as advantage) the communities that don't actively contribute/participate in farming take the advantage of having important functions like wedding in this month. For example, the business community prefers this month for wedding. It is usually the worst month for thriving businesses and recently this situation has changed a lot, as the businesses started providing discount shopping during that month. Each Friday of this month is set aside for prayer and worship.
And another important point to note that for the newly married couple is also an inauspicious month to sleep together. The reason being if the girl who is conceived in this month will deliver the baby in the month of May, the hottest month in Tamil Nadu (Agni natchathiram [pinezu] last 7 days of Chitharai and [munezu] first 7 days of Vaigasi) and it will be tough period for baby delivery.
The sixth month Purattaasi is auspicious to the effect that, most of the non-vegetarian Tamil people do not eat meat during the month. This faith can be considered much similar to the fasting by Muslims during the month of Ramadan. Each Saturday of this month is set apart to venerate the planet Saturn.
The full moon days and the new moon days have considerable importance among the Tamil people. One of the famous festivals, the Deepavali, is celebrated on the new moon day, in the seventh month Aipassi. The month of Aipassi is usually characterised by the North-East Monsoon in Tamil Nadu, which has given birth to a phrase, Aipassi Adai Mazhai meaning the "Non-stop Downpour".
The ninth month Maargazhi is characterised by winter in Tamil Nadu, and considered auspicious for maiden women to find their groom. The Shaivite fast of Tiru-vembaavai and the Vaishnava fast of Tiru-paavai are observed in this month.
The total number of days in a Tamil Calendar is an average 365 days and day's name are also similar to the western calendar. In temples, and for day to day purposes, Vakiya Panchangam is used. For astrological calculations Trikanitha Panchangam is used.