Makar Sankranti and why its date changes
Makar Sankranti is usually on 14 or 15 January, depending on the year, because it is the Sun's entry into Makara and the sidereal calendar drifts against the Gregorian one.
What it is
The sidereal solar year is not an exact number of days, so the entry moment falls at a slightly different time each year. A moment after midnight can shift the date to the next day, and different calendars apply different rules to fix the festival day.
How it is worked out or read
The site gives the sidereal Sun's entry in its calculations, but a festival date is set by the tradition's own rule, which is why printed calendars may disagree by a day.
Where it appears on this site
The Sun's position is in the birth chart. The sankranti article covers the general case.
What the date marks
Makar Sankranti is the moment when the Sun enters sidereal Makara. Because it is fixed by the Sun rather than the Moon, its civil date is nearly the same every year, which makes it unusual among Hindu festivals. At present it falls on 14 or 15 January. The exact day depends on the moment of the Sun's entry, the place, and the rule the community follows for an entry late in the day or after sunset.
Why it is not on the solstice
Makar Sankranti is associated with Uttarayana, the Sun's northward course, which astronomically begins at the winter solstice around 21 or 22 December. The two once nearly coincided, around the early centuries of the Common Era, when the sidereal and tropical zodiacs were close together. Since then the sidereal sankranti has drifted later by about one day every 72 years, so it now falls more than three weeks after the solstice. The festival keeps the name and the association, while the astronomical turning point has moved away from it.
The drift will continue
The drift is steady and predictable. A few centuries from now Makar Sankranti will fall around 16 or 17 January, and over thousands of years it will move through the calendar. This is a direct consequence of the choice to fix the zodiac to the stars, and it shows clearly how the sidereal year differs from the tropical year that the Gregorian calendar follows.
Rules for the day
Many traditions observe a punya kaal, an auspicious period around the moment of sankranti, for bathing and charity. If the sankranti happens late in the day or after sunset, many communities keep the festival on the following day. These rules differ between regions, which is why almanacs sometimes give different dates in the same year. For the local observance, follow the community's own almanac.
One festival, many names
The Sun's entry into Makara is celebrated across India under different names and customs: Pongal in Tamil Nadu, Uttarayan with kite flying in Gujarat, Magh Bihu in Assam, Lohri in Punjab on the eve, and Makar Sankranti or Sakraat in many other regions. Sesame, jaggery and new harvest grains are common to many of these celebrations, marking the festival's link with the agricultural year.