The Sun enters the Makara raasi (the zodiac sign of Capricorn - the goat), on Sankranti day, signifying the onset of Uttarayana Punyakalam.
The following has been translated from an article in Sadachara Vybhavam of the great Vaishnava devotee Sripad Madhwacharya:
Today is "Makara Sankranti" which is celebrated when the Sun begins the northward journey, marking the beginning of the "uttaraayaNa puNyakaalam". Sun enters the sign of "Makara", (Capricorn) from Cancer at this time. (This is also called equinox, when the day and nights are exactly the same). Starting this day, the duration of day starts to increase until "dakshiNaayanam" comes. Makara Sankranti signifies two things. One is the changing path of Sun and other is the beginning of "Uttarayana Punyakaala" Makara literally means "Capricorn" and sankranti means "change" or 'sankramaNa' literally means 'crossing'.
On this day, it is said that Sun passes from one Zodiac sign to another. A sankranti or sankramaNa marks the beginning of every solar month in the Vedic calendrical system when Sun passes from one sign to the other, it's the cusp of the months as astro-wallas call. But, being solar based the change of January 14th or 15th of every year is considered more sacred than any other month, as it is the beginning of Uttarayana Punyakaala.
Lord Surya on His dazzling chariot that has thousands of spokes and magnificent wheels, and driven by seven horses (seven colours of the spectrum) begins His journey northward into the Makara Rasi, filling our lives with light and getting rid of the darkness. This light not only fills our material life and helps our plants grow at seeding time, but also gets rid of our "internal" darkness, showing the path of self realisation - moksha.
On this day, devotees prepare a dish made of green grams, rice and jaggery (sarkkarai pongal) - representing Lord Narayana, Laxmi and Their Divine love. The festival reminds us that we need to establish that Divine Love within ourselves and give a direction to this "chanchala buddhi" and fix it our Lord Narayana, who brings light in our lives.
This same festival is celebrated as Pongal in the South of India. Harvest is brought home the new rice is then boiled in milk to make the sweet called "chakrapongali", the special cuisine of the eve. This harvest festival is celebrated for 3 days. The delicacy is first offered to Sun God, the chief deity under worship, that is later distributed to all as prasadam. The harvest is celebrated as being blessed with Dhanyalakshmi and a splendid feast is shared by all after the Ishta Devatha pujas.
Traditionally, this period is considered an auspicious time and the veteran Bhishma of Mahabharata chose to die during this period. Bhishma fell to the arrows of Arjuna. With his boon to choose the time of his death, he waited on a bed of arrows to depart from this world only during this period. Sri Bhishma Pitamaha waited on the bed of arrows till the "uttaraayaNa punyakaalam" has arrived, and then breathed once more, for the last time. It is believed that one escapes the cycle of repeated birth and death, if one gives up one's body on this day. The uttarAyaNa (norther course of the Sun) was traditionally considered more auspicious than the dakshiNAyana (southern course of the Sun). Over the years, this led some to believe that dying during dakshiNAyana, the southern path of the sun, was inauspicious and did not lead to moksha - release - liberation.
This belief is cast aside in the Brahma-Sutras that the time of death is immaterial in the case of the intelligent devotee. In those Sutras 4.2.19-20, it is established that one who has knowledge of the Lord can die during any time; the exaltations of uttarAyaNa are merely to praise meditation on the path to Vaikuntha, which the wise devotees jIva takes upon death. This divine path beginning with light, is presided upon by deities of greater and greater brilliance, with uttarAyaNa being one of them. This marga - path culminates in Vaikuntha, the state of moksha, and shastra says it should be meditated upon daily by the yogi of devotion.
Sesame oil is used in making sweets and dishes, particularly on this day. It is believed that sesame seeds are used for purificatory purposed (or as being auspicious - "tila juhomi sarasaagum sapishhThaam gandhaara mama chitteramanutu swaaha", "tila krishnaa tila shevataa tila saumyaa vashaanugaa"...).
So generally special offerings of til (sesame) rice and corn (first harvest) are offered and distributed, especially preparations of rice kitcheri with sesame and corn are a prefered offering to the Lord on this day. In different parts of India slight variations (gauna - local traditions prevail) but the gereral observance is much the same. In the South of India they absorb themselves in the festival of Pongal - new harvest festival.
Some devotees also have elaborate festivals offering six kinds of sesame preparation in the service of the Lord on Sat-tila ekadasi:
Other information about Makara sankranti - lincluding local traditions - festivals:
The wide range of festivities can better be witnessed by one along the countryside than in the urban localities. The festivities last the whole of Dhanurmasam, ending with Sankranti.
Gobbemmalu – Young girls are seen to put up Gobbemmalu. These are round balls of cow dung that are decorated with vermilion, turmeric and coloured flowers. These are placed in the middle of the bright muggus in the front yards of the dwellings and are worshipped with Navadhanyas (nine types of grains) to welcome the Lakshmi, the Goddess of abundance and prosperity. Spinsters worship for good husbands on the occasion. After this, the women folk get gathered to sing and dance to the accompaniment around these gobbis.
Many a traditional folk form add glory to the festive scenario. Haridasulu (Madhwas) are seen singing eulogies of the Almighty and seeking arms from the people in the early hours of the days. Gangireddu Ata, is another festival where trained oxen dance to the tunes of the Nagaswaram played by their master is a great attraction. Kommudasarulu & PittalaDora are other folklore that brighten the days with their healthy humour.
Kites – kites are seen soaring high in the sky on the day of Makara Sankranti. This is symbolic of the existence of mankind, with the kites depicting the human lives and the manza depicting the eternal bond between us and the divine self. People however seem to correlate the soaring kites with their high spirits on the eve of Makara sankranti. Competitions held with kites of all sizes and shapes and the sharp edged manza draw people on the terraces and in open grounds to witness the riot of colours - fun for the gurukulis.