Uttarāyana

Uttarāyana (उत्तरायण) is the six month period between Makar Sankranti around (January 14) and Karka Sankranti around (July 14), when the Sun travels towards north on the celestial sphere. The period from July 14 to January 14 is known is Dakshināyana (दक्षिणायण). The starting of Uttarayana is celebrated as Makara Sankranti throughout India, which is declared as National festival by Government of India.

Makara Sankranti (It is a sanskrit word) is the harvest festival of India. Sankranti means transmigration of Sun from one Rashi (something akin to the zodiac) to the other. Hence there are 12 such Sankrantis in all. But the transition of Sun from Dhanu Rasi to Makara Rasi marks the starting of Uttarayana which means northern movement of Sun. Since Uttarayana is Considered as auspicious time, Makara Sankranti is celebrated as the beginning of that period.
It is celebrated by many cultures in different manners.
In North India,
Punjab- Lohri
West Bengal and Assam - Bhogali Bihu
Gujarat and Rajastan - Uttarayan (Kite flying festival)
In South India,
In Kerala and Tamilnadu and Tamils in Sri Lanka - Pongal
In Andhra Pradesh - Sankranthi
Other places as Makara Sankranti.
The Gujarati people are the most prominent in celebrating Uttarayana, which is a prominent part of Gujarati culture by flying kites all over Gujarat and in Bombay, India. Sankranthi is the most important festival of Telugu people, like Deepavali is for Tamil people.