Makara Sankranti is one of the most celebrated festivals in India but astrologically, it is the day when Sun begins its movement away from the tropic of Capricorn and towards the northern hemisphere.
That is why we notice Sun rising and setting slightly towards North-East and North-West respecitvely between mid-january to mid-july in every year.
This period is called Uttarayan in vedic terms and it means uttar(north)+ayan(movement) of sun.
The remaining 6 months are termed as dakshinayan which means south movement of sun.
Originally, this was celebrated as Winter Solstice in ancient India but later due to the fact that the solstices are continually precessing at a rate of 50″ / year due to the precession of the equinoxes, winter solstice now occurs at 21st december each year.