Makar Sankranti or Uttarayan or Maghi or simply Sankranti, also known in Bangladesh and West Bengal as Poush Sankranti, and in Nepal as Maghe Sankranti, sam(n)kranti here means ‘transfer’, this day is considered as the transition day of Sun into the Capricorn. Now the sun moves northwards in the Hindu calendar, dedicated to the deity Surya (sun), many native festivals are organised all over India. It is observed each year the day Sun enters the Capricorn zodiac which corresponds with the month of January as per the Gregorian calendar. It marks the first day of the sun's transit into Makara rashi (Capricorn).Due to the addition of one day in leap years, the date of Makar Sankranti may vary a bit. On leap years it falls on 15 January, otherwise on 14 January. There are 365.24 days in one year but we can use only 365 whole days. Then we add one day on the leap year. By the time of leap year, year Calendar is lagging almost one day behind the sun, causing Makar Sankranti to fall on 15 January. When correction is made Makar Sankranti falls back on 14 January.
The festivities associated with Makar Sankranti are known by various names Magh Bihu in Assam, Maghi in Punjab, Maghi Saaji in Himachal Pradesh, Maghi Sangrand or Uttarain (Uttarayana) in Jammu, Sakraat in Haryana, Sukarat in central India, Pongal in Tamil Nadu, Uttarayan in Gujarat, and Uttar Pradesh, Ghughuti in Uttarakhand, Dahi Chura in Bihar, Makara Sankranti in Odisha, Karnataka, Maharashtra, Goa, West Bengal (also called Poush Sankranti), Uttar Pradesh (also called Khichidi Sankranti), Uttarakhand (called as 'Uttrayni') or as Sankranthi in Andhra Pradesh and Telangana, Maghe Sankranti (Nepal), Songkran (Thailand), Thingyan (Myanmar), Mohan Songkran (Cambodia), and Shishur Saenkraath (Kashmir).On Makar Sankranti the Sun god is worshipped along with Lord Vishnu and goddess Lakshmi throughout India.
Makar Sankranti is observed with social festivities such as colourful decorations, rural children going house to house, singing and asking for treats in some areas,melas (fairs), dances, kite flying, bonfires and feasts. The Magha Mela, according to Indologist Diana L. Eck, is mentioned in the Hindu epic Mahabharata. Many observers go to sacred rivers or lakes and bathe in a ceremony of thanks to the sun. Every twelve years, the Hindus observe Makar Sankranti with Kumbha Mela – one of the world's largest mass pilgrimage, with an estimated 60 to 100 million people attending the event. At this event, they say a prayer to the sun and bathe at the Prayagaraj confluence of the River Ganga and River Yamuna, a tradition attributed to Adi Shankaracharya.
Reference :- https://en.wikipedia.org/wiki/Makar_Sankranti