Whenever The Sun enters a sign, that day is called Sankraanti (transition). So although Sankraanti comes every month, because The Sun transits to another Sign every month, this is Makar Sankraanti and is supposed to be of greater importance than other Sankraanti. As it is known, that Earth's orbit is divided into twelve Signs, and The Sun passes through all the Signs in one year. Which means that He stays in one Sign for one month only. On this day The Sun enters the Makar Raashi (Capricorn Sign). 

This is the only Hindu festival which does not fall on the day of Hindu Calendar. It always falls on January 14th when Sun enters the Makar Raashi (Capricorn Sign) and becomes Uttaraayan (coming towards North) till about 14th July when The Sun again starts going towards south - a period of six months.

It is supposed to be an auspicious day and is celebrated with lots of donations and cooking Khichadee (a dish made with Moong Daal (split lentils), rice, clarified butter and spices) in the Indian houses. Sesame seed sweets with sugar or jaggery are made, eaten and donated to Braahman.

This occasion is considered such auspicious that people think whosoever dies during this Uttaraayan period goes to Heaven directly. Bhishm of Mahaabhaarata waited for this period to come to die.

This festival is celebrated in almost whole India in one or the other way. In Southern part, it is called Pongal, in Punjab it is called Lohdee, in Maharashtra it is called Til (sesame seeds) Sankraanti.