Today is Maghi (or some variant), in many parts of the Indian subcontinent and SE Asia. It is an annual festival on the first day of the month of Magha in the Bikrami calendar, when the sun enters the sign of Makara or Capricorn. The eve of Maghi is called Lohri (https://www.bookofdaystales.com/lohri/ ). It is one of the seasonal gatherings of the Sikhs, and is celebrated at Muktsar in the memory of forty Sikh martyrs (Chalis Mukte), who once deserted the tenth and last human Guru of Sikhism, Guru Gobind Singh at Anandpur Sahib, but later rejoined the Guru and died while fighting the Mughal Empire army led by Wazir Khan in 1705. Sikhs make a pilgrimage to the site of this Sikh-Muslim war, and take a dip in the sacred water tanks of Muktsar. A fair (mela) called the Mela Maghi is held at Muktsar Sahib every year in memory of the forty Sikh martyrs. Before this tradition started to commemorate the Sikh martyrs, the festival was observed and mentioned by Guru Amar Das, the third Guru of Sikhism.
Makar Sankranti (or Pongal) is celebrated on this date in other parts of the Indian subcontinent by Hindus, always on the first day of the month of Magha in the Bikrami calendar. Hindus bathe in the Ganges or if that is not possible, in some other river, rivulet, canal or pond.
Maghi is celebrated by eating kheer such as roh di kheer which is an old dish in which rice is cooked in sugarcane juice. The dish is prepared in the evening before Maghi and is kept overnight to cool. It is served cold next morning on Maghi with red-pepper mixed curd. In some parts of Punjab, it is also traditional to eat kichdi (rice and moong beans) mixed with lentils, or raw sugarcane and jaggery.
Here’s a great video on preparing roh di kheer in a traditional Punjabi kitchen: