"Uttarayan", means the day on which sun moves towards "Uttar" means North direction.
We celebrate it only on 14 th , the next day 15
th is called "Vasee Uttarayan" means the following day of "Makar Sankranti" or "Stale Uttarayan".
It is considered that sun marching towards North takes souls to Heaven and heading towards south takes souls to hell.
In Mahabharata it is mention when Bhishma decided to leave body on the day of Uttarayan only on the advice of Lord Krishna.
Hindu calendar is based on moon, but this day is related with sun.
And as per Gregorian calendar this day is found that the sun enters the Capricorn (Sanskrit:
Makar) zodiac , so Makar Sankranti is celebrated on this day only as it is solar event only.
Hindu calendar is moon event based
calendar and Gregorian Calendar is strictly a solar calendar based on solar events of 365 days.
As Makar Sankranti is solar event it is followed by Gregorian Calendar.
hinduism.stackexchange.com/users/577/prem30488
No, it is not celebrated according to the English calendar.
From the point of view of Jyotisha , Makara Sankranti is celebrate when Surya (Sun) enters Makara Rashi (the zodiac named Capricorn).
As, Jyotisha follows a sidereal (moveable) zodiac, these dates can change i.e. Surya can enter Makara on a different date.
The question itself says that it can fall either on 14th or 15th.
The English calendar follows a tropical (fixed) zodiac, where they assume that Sun enters Capricorn on the same date every year which is not in sync with the facts (only an assumption).
This article can be referred to learn more.
Hope this helps! :)
hinduism.stackexchange.com/users/4157/Amit Saxena
A simple logic: Who gave English Calendar to Bhishma Pitamaha to leave his body on Makara Sankranti?
Bhishma Pitamaha left his body on the day of Makara Sankranti.
He wishfully remained alive up to the day of Makara Sankranti.
The Mahabharata, Book 13:
Anusasana Parva: Section CLXVII The Mighty Bhishma then, taking the strong hand of Yudhishthira, addressed him in a voice deep as that of the clouds.
That thorough master of words said, 'By good luck, O son of Kunti, thou hast come here with all thy counsellors, O Yudhishthira!
The thousand-rayed maker of day, the holy Surya has begun his northward course.
I have been lying on my bed here for eight and fifty nights.
Stretched on these sharp- pointed arrows I have felt this period to be as long as if it was a century.
O
Yudhishthira, the lunar month of Magha has come.
Thus, Makara Sankranti is not by English Calendar.
It falls when the Sun Enters the Capricorn (Makara) constellation.
It usually happens in January 14th/15th
but it doesn't mean that it is calculated by English Calendar.
It isn't always necessary that Makara Sankranti falls in January 14th/15th.
hinduism.stackexchange.com/users/5154/Tejaswee
It doesn't necessarily comes on 14 th January, We have seen it almost falls on 14 January
many times it comes earlier or many times later.
Hinduism is the major festivals of Makar Sankranti.
This festival is celebrated all over India in some form.
Sun in Capricorn occurs on Poush Mass is celebrated on the solstice.
This festival is celebrated mostly on fourteen of January.
Sometimes the festival twelve, thirteen or fifteen may be, it depends on the fact that when the sun enters Sagittarius Capricorn left.
The day begins with the sun's solstice speed and for this reason it is also called Uttarayani.
hinduism.stackexchange.com/users/834/Keshav
We follow Moon Calendar as per that the Sankranti comes when Sun Enter Makar (Capricorn) which is also the start of Uttarayan.
The English Calendar based on Sun, therefore we find its always on 14th January.
But as per Moon Calendar it keeps varies....
The Calendar which we follow now a days is Gregorian Calendar, which is based on Sun so its always fixed to 14th Jan.
For Sun based Calendar, Moon based festival keeps varies and vice as versa.
hinduism.stackexchange.com/users/13615/MANISH PARGANIHA
The Hindi Sanskrit text above translates to the following: Hinduism is the major festivals of Makar Sankranti.
This festival is celebrated all over India in some form.
Sun in Capricorn occurs on Poush Mass is celebrated on the solstice.
This festival is celebrated mostly on fourteen of January.
Sometimes the festival twelve, thirteen or fifteen may be, it depends on the fact that when the sun enters Sagittarius Capricorn left.
The day begins with the sun's solstice speed and for this reason it is also called Uttarayani.
hinduism.stackexchange.com/users/860/Brian Rose
Why does Makar Sankranti always happens to be on Jan 14 th  / 15 th , while all other Hindu festivals depend on size of moon like Chaturthi, Amavasya, Poornima etc.  Are there any more Hindu festivals which also follows English calendar?
Why does Makar Sankranti follow English calendar?
hinduism.stackexchange.com
<festivals>
hinduism.stackexchange.com/questions/3132
hinduism.stackexchange.com/users/804/Nikhil Vidhani
The Calendar which we follow now a days is Gregorian Calendar, which is based on Sun so its always fixed to 14th Jan.      The Calendar which we follow now a days is Gregorian Calendar, which is based on Sun so its always fixed to 14th Jan.    
The Calendar which we follow now a days is Gregorian Calendar, which is based on Sun so its always fixed to 14th Jan.    