Makar Sankranti - Festival of friendship

In India, in Hinduism, Every festival, every social custom has science behind it.
Makar Sankranti marks the beginning of the Sun's journey to the Northern Hemisphere called Uttarayana.
Makar stands for the constellation Capricorn and the Sankranti of a month is the day when the Sun passes into a new rashi (zodiac).
The day and night on Makar Sankrant are of exactly of equal hours.

Makara Sankranti is also celebrated throughout India as a
harvesting festival.It is a way of giving thanks to the elements of
nature. In the coastal regions, it is a harvest festival
dedicated to Indra.

Celebration all over INdia:
In Maharashtra, when two persons greet each other on this festive day, they exchange a few grains of multi-coloured sugar (TilGul) and say "til-gul ghya, god god bola" means "accept these tilguls and speak sweet words".(hereafter,let there be friendship between us).
In Gujarat, perfect weather conditions at this time of the year mean this day is celebrated as Patang Divas (Kite Flying Day).
In Tamil Nadu and Andhra Pradesh, festival called as Pongal,the whole festival lasts for four days, the first day Bhogi, the second day Sankranti, the third day Kanuma and the fourth day, Mukkanuma.
In Assam, the festival is celebrated as Bhogali Bihu.
In Punjab it is called Lohri.
In Uttar Pradesh, Sankrant is called "KICHERI".
In Bundelkhand and Madhya Pradesh this festival of Sankrant is known by the name "SUKARAT" or "SAKARAT".

On this day people eat "bajari" bread mixed with "til" (Sesamum). On the feast of Sankrant "til" is
given great importance, for in this season it is considered to have special nutritive and medicinal qualities. "Til" is a very oil-giving seed. Mixed with jaugari or sugar it becomes a very sticky
sweet which people exchange with one another as a sign of friendship. "Tilgul ghya, god bola." ("Take tilgul and speak sweetly") is the phrase on everybody's lips.
Til-Gul is heat giving taht significant in winter days.

Throughout the year the sun passes through twelve constellations: Mesh (Ram, Aries), Vrishabh
(Bull, Taurus), Mithun (Couple, Gemini), Kark (Crab, Cancer), Sinh (Lion, Leo), Kanya (Girl, Virgo), Tula (Balance, Libra), Vrishchik (Scorpion), Dhanu (Bow, Saggitarius), Makar (Crocodile, Capricorn),
Kumbh (Wateijar, Aquarius), Min (Fish, Pisces).
When the sun does not cross any constellation then there is an extra month called "Adhik Mas". The crossing of the Makar constellation takes place in the month of Paush.

sources:

http://www.indiamike.com/india/chai-and-chat-f73/makar-sankaranti-pongal-greetings-t9234/
http://www.cuhcs.org.uk/sanskaar.article.php?article=makar&year=2004