Makkar Sankraanti or Maghi, is a festival in the Vedic calendar dedicated to the deity Surya i.e., the Sun.
In most years it is celebrated on 14th January. Each year, it is observed in the lunar month of Magha which corresponds with the month of January as per the Gregorian calendar and is a day the Hindus, Jains, Sikhs, Buddhists and other people of Vedic Paramparaa celebrate their harvests. It marks the first day of the sun’s transit into Makkar Raashi i.e., Capricorn, marking the end of the month with the winter solstice and the start of longer days.
Sankraanti is a Sanskrit word and one of the meanings is ‘The actual passage of the Sun or other planetary bodies from one sign of the Zodiac into another’.
The earth, during its orbit around the sun, spins on its axis, each rotation lasting 6 months – during these 6 months, one hemisphere nears just a little towards the Sun; during the next 6 months it inches just a little away from the Sun. Hence the lengthening of day vs night (and whether it is winter or summer) is experienced in an opposite fashion in the northern (Uttaraayana & observe Makkar Sankranti) and southern (Dakshinayan observing Karka Sankraanti) hemispheres.
Therefore the festival has spiritual as well as astronomical significance.