Gangasagar Mela is held during the auspicious period of Makar Sankranti. The Hindu calendar is luni-solar calendar where adjustments are made regarding lunar and solar cycles to keep track of seasons. This is why the date of Gangasagar Mela usually varies between 14th and 15th January each year.
The first day of the solar month is Makara, when the sun is believed to be entering the Makar rashi phase, which again corresponds with the sun sign, Capricorn. The other word, Sankranti, refers to the transition of the sun from one phase to the other. There are 12 Sankrantis in total, of which Makar and Karka are of massive significance in terms of religious beliefs.
The climate also undergoes remarkable change during this time. Held soon after winter solstice, this is the time when days finally start to get longer and nights get shorter. This reflects an intrinsic tie between Jyotisha practices and astronomical movements. Since Vedic times this ancient system of knowledge has been a guiding light for an entire civilization and what is awe inspiring is the fact that it remains relevant even to this day!