Pongal is a harvest festival celebrated by Tamil people at the end of the harvest season. Pongal is a four day festival which usually takes place from January 13 to 16 in the Gregorian calendar i.e., the last day of the Tamil month Maargazhi to the third day of Tamil month Thai.
The second of the four days i.e., first day of the Tamil month Thai is the second and main day of the festival and this day is known as Thai Pongal. This day coincides with Makara Sankranthi which is a winter harvest festival celebrated throughout India. The day marks the start of sun’s six-month long journey northwards or the Uttarayanam. This also represents the Indic solstice when the sun purportedly enters the 10th house of the Indian zodiac i.e. Makara or Capricorn.
Information from wikipedia.com