How the date is fixed
The reference day is the day the Sun enters sidereal Makara. It is a solar festival, so its Gregorian date stays nearly the same every year. Lohri itself is kept 1 day before that day.
About
Lohri is kept on the evening before Makar Sankranti, usually 13 January, and so follows the solar calendar. It closes the winter sowing season in Punjab and the surrounding plains. The date is found by computing the Makara sankranti and taking the previous evening; the engine lists it against the sankranti day with the note that the observance is on its eve. Folk songs about Dulla Bhatti and the first Lohri of a newborn or a newly married couple are part of the custom.
How it is observed
Families gather round a bonfire after dusk and offer sesame, jaggery, popcorn and peanuts to the fire.