Sankranti or Sankranthi marks the transition of the Sun into Makara rashi (Capricorn) on its celestial path. Traditionally, this has been one of many harvest days in India. Makar Sankranti festival, unlike other Hindu festivals, is not dependent on the position of the moon, but on position of the sun. Makar Sankranti is perhaps the only Indian festival whose date always falls on the same day every year i.e. the 14th of January.
This festival is celebrated for innumerable reasons and in innumerable ways depending on the climate, agricultural environment, cultural background and location.
A well-known reference of this day came when the great grand-sire of Mahabharata fame, Bhishma, declared his intent to leave his mortal coil on this day. He had the boon of Ichha-Mrityu (death at his will) from his father, so he kept lying on the bed of arrows till this day and then left his mortal coil on Makar Sankranti day.
It is believed that the person, who dies during the period of Uttarayana, becomes free from transmigration (rebirth). So this day was seen as a definite auspicious day to start a journey or endeavours to the higher realms beyond.