Makar Sankranti is a religious festival with social, astrological and geographical significances. It is celebrated on the first day of the solar month Makara at which the Sun enters Makara rasi (Capricorn). Unlike most of the celebrations, Makar Sankranti is observed according to the solar cycle. It is also regarded as a harvest festival dedicated to the Sun God Surya and is celebrated all over India with great zest and devotion.
The term Sankranti refers to the transit of the Sun from one sign to other. The first day of every solar month is a Sankranti signifying the transit of Sun to a new sign. Thus, there are 12 Sankrantis out of which Makar Sankranti and Karka Sankranti are given more importance for their astrological and climatic significance. Makar Sankranti comes after the winter solstice in Dhanu and Karka Sankranti comes after the summer solstice in Midhuna.
Once the Sun passes the equinox in Thula, the days get shorter than the nights until reaching the winter solstice in Dhanu. On the winter solstice, daytime will be the shortest of the year, after which, the Sun starts its movement towards the north. Once the Uttarayana or the northern movement starts, the duration of daytime increases.
Thus, the Makar Sankranti festival marks the end of the month with the winter solstice and the end of long nights. With Makar Sankranti or the beginning of the month of Makara, the days gets longer and warmer consistently.