The Thiruvalluvar Day is celebrated annually on January 15th or January 16th in Tamil Nadu as a part of Pongal (or Sankranti) celebrations. This year, it falls on January 15th. The day is so named to honour the contribution of the 4th century AD Tamil poet Thiruvalluvar, who is best known for his literary work, the Thirukkural (Sacred Couplets). 

Thirukkural: A beautiful collection of moral aphorisms

Written in Tamil, the Thirukkural is a beautiful collection of 1330 couplets divided into 133 adhigarams (sections, ) and is considered one of the greatest works of Tamil literature. The work is divided into three books and dates from 4th century though some scholars would put it much earlier. 

Thirukkural deals with on ethics, political and economy and love. 

Why is the Day Celebrated on January 16 or January 17?
The resolution to celebrate Thiruvalluvar Day by Kali Sivakannuswami Pillai and Padmashri Suppayya was passed on January 17, 1935. The first Thiruvalluvar Day was celebrated in May 1935. However, over time the celebrations began to fade away. In 1954, an Elam scholar from Sri Lanka took efforts to revive the day. Today, the day is widely celebrated in Tamil Nadu, Sri Lanka, and Myanmar.

Thiruvalluvar was a Sangam-era poet. 

Thiruvalluvar Day is observed as part of Pongal celebrations in Tamil Nadu to honour the great Tamil poet and philosopher. 

Thirukkural is the most famous of the Pathinenkeezh Kanakku ‘ nools ’ (Eighteen Literary Works).