This is a popular Hindu festival that occurs on or around 14 January across India. It is also celebrated as a public holiday in Sri Lanka.
The day is known by various names and a variety of different customs are observed in the different Indian states.
Despite these variations, it is a harvest and thanksgiving festival marking the start of spring, the end of the traditional farming season and the gathering of the first food from the harvest.
It is unique among Hindu festivals as the date is based a solar calendar rather than the phases of the moon. This means it falls on 14 January in the western calendar. In 2016, it will be celebrated on 15 January in Karnataka, Telangana and Tamil Nadu.
In 2016, Thai Pongal will be celebrated in Sri Lanka on 15 January.
The date of Pongal marks the start of Uttarayana, the time when the sun starts to move northwards after the winter equinox.
This date is now actually 21 December in the Western Calendar, but the time of the equinoxes moves by 50 seconds each year due to the wobble of the Earth's axis.
This shows the ancient age of this festival - a thousand years ago, the festival was on 31 December.
Uttarayana is also considered a time of good fortune and important events are scheduled during this period.
The festival is celebrated for up to four days and is known as Pongal.
The harvest festival is known as Pongal and lasts for three days.
The second day is known as Pongal day. This is the key day for celebrations and holidays in many states. To mark a good harvest, milk or rice is cooked until they boil over - 'Pongal' means 'it boils'. The food is offered to the gods (including the sun or rain gods, depending on the region's climate) before people eat it to cleanse themselves of past sins.
The third day is Mattu Pongal (festival of the cow). It is a day to offer thanks to the village cows and oxen, who played a key role in the season's farming as they are used to plough the land. The cows and oxen are bathed, decorated with garlands and worshipped.
In southern India, all three or four days of Pongal are considered important. Southern Indians who have settled in the north will usually celebrate the second day. As it coincides with Makara Sankranti in the north, it may also be called Pongal Sankranti.
Here the day is known as Makar Sankranti. Makara Sankranthi refers to the event of the Sun entering the zodiac sign of Makara or Capricorn, marking the start of its northward journey. It is a festival of the young and the old.
In Ahmedabad in Gujarat, a large part of the celebration is the International Kite Festival. The skies are filled with kites, and kite makers come from many other cities to make and fly multicoloured kites in all design and sizes. At night, kites with paper lamps filling the sky with light.
The festival is called Lohri. December and January is winter in Punjab and bonfires are lit on the eve of Sankranti. Sweets, sugarcane and rice are thrown on the bonfires and friends and relatives gather together.
The period is celebrated as Kicheri. An important tradition on this day is to have a bath and masses of people can be seen bathing in the Sangam at Prayagraj where the rivers Ganga, Jamuna and Saraswathi flow together.