Most of the Panchang calendars in India, like the Punjabi calendar are calibrated against the monthly full moons and the monthly transitions of Sun from one one constellation to the next. In other words, these calendars contain Lunar months and Solar Months. The first day of the solar month with the name Makara would have coincided with the Winter solstice around the third century CE. The ancient astronomers in India were aware of the astronomical phenomenon called "Precession" and precisely timed Vedic fire rituals to the Winter Solstice.
Ancient Indians knew that the winter solstice day slips a day with respect to the solar months in seventy years. Certain Vedic rituals were discontinued around the turn of the common era however the memory of their celebrations appear to have survived through folk lore. Almost every part of India has a festival similar to Lohri in Punjab such as Pongal, Bhogi and Sankrant. But the folk lore and timing conventions around Lohri suggests that the festival was known even earlier ear.
The day following Lohri is called Maghi Sankrant in Punjab. This day is called Makara Sankrant in the rest of India. Makara is the Sanskrit name for the Capricorn contellation. Thus Makara Sankrant is the day when the Sun enters Capricorn. Sun's monthly transitions were named after the lunar month associated with that 30 day period during an earlier era. For example, Makara Sankrant would have been called Poush Sankrant during earlier times. Interestingly Lohri is associated with Magh Sankrant which is a month after Poush Sankrant. This suggests that the winter solstice had already slipped by a month by the third century CE when Makara Sankrant got established as a festival in the rest of India.
It takes 2800 years for solstice to fall back a full Hindu calendar month. One can therefore conclude Magh Sankrant being the day after Lohri that Lohri was celebrated during 2500 BCE. Was Lohri then celebrated during the time of the Harappan civilization?