
The northern hemisphere now tilts towards the sun and the rays of the sun appear directly over the Tropic of Cancer.From our perspective on Earth, the sun has apparently traveled between the Tropic of Capricorn to the Tropic of Cancer, Uttaryana.
The Rishis, scientist sages, observed that longer daylight after the winter solstice in the northern hemisphere due to the apparent movement of the sun north, uttara, towards the tropic of Cancer brought with it warmth, light, productivity, agricultural bounty and prosperity. This must have been an extremely significant aspect of their lives as they lived through the chill and darkness prevalent during the Ice Age in the Treta Yuga.Therefore this was certainly an occasion to celebrate.
Uttaryana begins with the winter solstice. .The day is marked as punyakalam and is considered as Day of Devas.
Uttaryana, then and now, means the end of the days of darkness in the northern hemisphere. The six months of darkness at the north pole ends and the six months of light begin. These six months of human time are considered to be one "day" of the Devas. The Devas awaken from their slumber and shower their blessing on us. The first month of Uttaryana, the pre-dawn of the devas, we spend euologising the Gods, with song, and prayer. We line our streets with the festive kolams as welcome to this wonderful period as we publicly display our joy. We strive to celebrate all that is important during this time like the viviha and the upanayana. We begin new ventures with the hope that the Devas witness these life passages of ours and shower their blessings on our efforts. Vaikunta Ekadashi, or Moksha Ekadashi is during this period, the dawn of the devas, when Lord Vishnu awakens to embrace his devotees and grant them moksha from the cycle of rebirth
As described in Bhagavat Geetha..
http://www.bhagavad-gita.org/Gita/verse-08-24.html
Makar Sankaranti is one of the highly auspicious days in a Hindu calendar, dedicated to the worship of Lord Sun. On this day the Sun enters into the zodiac Capricorn. It is also known as Uttarayana Punyakalam. The day time of Devas begins with the Makar Sankranti and lasts for six months. Days and nights both are of equal length on 14th January. Makar Sankranti is the first day of Uttaraayana. During six months of the year, the days are longer than the nights. Summer season begins due to the movement of Sun towards northern hemisphere. Heat and light of Sun starts increasing. As a result of this, there is a development of awareness and energy among the creatures.
According to the Mahabharata, Bhishma... who lay wounded on a bed of arrows is said to have been waiting for the period of Uttarayan to set in, in order to breathe his last since It was believed that the person who dies on this auspicious day of Sankaranti escapes the cycle of birth and re-birth and that his soul mingles with the Almighty.
lord Shiva had showered his blessings on his devotees, especially on sages, on this day. Some people believe that lord Shiva had donated self-knowledge to lord Vishnu on this day.
Apart from this, it is also believed that Sun goes to meet his son Saturn on this day. This meeting of Sun and Saturn is celebrated as "Makar Sankaranti".
on this day, Holy Ganga followed Bhagirath to reach the hermitage of saint Kapil and had fallen into the ocean. That is why, bathing in river Ganga and making donations on shrines is considered especially significant.
And as The Ganga is holy river to Hindus and all Religions based in India
The day is marked as punyakalam.And is considered as Day of Devas.
While Dakshinayan considered as night when devas go to sleep.