Why is Makar Sankranti always on the 14th of January?

Makar Sankranti falls on the day of the year when the sun-considered the king of all grahas (planets)-is in the rashi (zodiac sign) known as Makar (Capricorn). This is considered the most beneficial and auspicious zodiac of the sun. The calculations for determining Makar Sankranti are done according to the solar calendar. Therefore, Makar Sankranti always falls on the 14th January according to the English calendar. It is usually the month of Magh of the Hindu calendar, the 'Tithi' or the position of the moon keeps shifting because of the difference in calculations.

In the north, this festival is called Lohri: The Bonfire Festival
It is celebrated by both children and adults. In the morning on Lohri day, children go from door to door singing and demanding the Lohri 'loot' in the form of money and eatables like til (sesame) seeds, peanuts, jaggery, or sweets like Gajak, rewri, etc. They sing in praise of Dulha Bhatti, a Punjabi avatar of Robin Hood who robbed the rich to help the poor, and once helped a miserable village girl out of her misery by getting her married off like his own sister.
The Bonfire Ritual - In the evening, with the setting of the sun, huge bonfires are lit in the harvested fields and in the front yards of houses and people gather around the rising flames, circle around (parikrama) the bonfire and throw puffed rice, popcorn and other munchies into the fire, shouting "Aadar aye dilather jaye" (May honor come and poverty vanish!), and sing popular folk songs. This is a sort of prayer to Agni, the fire god, to bless the land with abundance and prosperity. After the parikrama, people meet friends and relatives, exchange greetings and gifts, and distribute prasad (offerings made to god). The prasad comprises five main items: til, gajak, jaggery, peanuts, and popcorn. Winter savories are served around the bonfire with the traditional dinner of makki-ki-roti (multi-millet hand-rolled bread) and sarson-ka-saag (cooked mustard herbs).
Song & Dance - Bhangra dance by men begins after the offering to the bonfire. Dancing continues till late night with new groups joining in amid the beat of drums. Traditionally, women do not join Bhangra. They hold a separate bonfire in their courtyard orbiting it with the graceful gidda dance

In the south, Sankranti becomes Pongal, a harvest festival.
Cows and bulls are decorated and taken in procession around villages. The first rice of the new harvest is ritually offered to the sun god and cooked in different ways to symbolise plenty. The food cooked for such feasts is also offered to the cows on that day. The special sweets made on this occasion are Sakkami Pongal or rice cooked in jaggery and Ven Pongal or rice cooked with green gram, nuts and ghee. The season of Sankranti ends with Ratha Saptami, the seventh day of the bright half of Magh, when the sun and his golden chariot are honoured.

West Bengal - last day of the Bengali month of Poush
Thousands of pilgrims from different parts of the country gather at Gangasagar, the point where the holy river Ganges meets the sea, to take a dip and wash away all the earthly sins.

Magh Bihu or Bhogali Bihu is Assam's one of the most important festivals.
Cutting across the bars of class and caste, it celebrated by all and sundry. Magh Bihu or Bhogali Bihu (derived from the word 'Bhoga' meaning eating or enjoyment) is celebrated when the harvesting is over. It is a harvest festival.
On the eve of Bihu day, called "Uruka", women prepare rice cakes and other refreshments and young men build a temporary shelter in the open, collect firewood, often by stealing, which is permissible on this occasion, for a bonfire.

The most significant part of this day is the building of 'Meji' - a structure built of logs of wood placed in pairs, tier above tier till they rise to considerable heights and present the appearance of a lofty temple. The whole night is spent in feasting, merry - making dancing and singing.

The half-burnt sticks and ashes of the meji are strewn on the fields and at the root of the fruit trees as they are believed to increase fertility.

In Gujarat and the other western states, Uttarayana
The change in the direction of winds at this time of the year. It is marked by thousands of colourful kites which dot the clear blue sky. Young men vie with each other to win community kite-flying competitions and then come home to a special winter feast in the evening. In these states, January is a month for eating newly-sprouted vegetables, sweets made of milk and fruits of the season.

ln Maharashtra, Karnataka as well as parts of Andhra, Makar Sankranti
A day of goodwill and friendship. Sesame chikki ladoos and sugardrops are distributed by everyone as a symbol of the need to be generous and kind to everyone Women wear new clothes, new glass bangles and hold get-togethers to share sweets and gifts. A new bride is given ornaments made of sugardrops and her new relatives are invited to meet and welcome her at Haldi Kumkum celebration. In rural Maharashtra, Sankranti brings in feasts when tender Jowar is eaten with salt and lemon juice, as well as fresh vegetables, guavas, custard apples, grapes, oranges and other fruits of the season.