Makar Sankranti (मकर संक्रांति)

Makar Sankranti (मकर संक्रांति) is a festival to celebrate the transition of Sun from Sagittarius (धनु) into Capricorn (मकर), as per the sidereal zodiac or the visible zodiac. It is a transition point marking the beginning of longer daylight in the northern hemisphere. The sun appears to travel northwards; hence the festival is also known as the northward journey (उत्तरायण). Vishnu Puraan (2.8.28) states that in the beginning of its northward journey, the Sun enters Capricorn. The festival is celebrated around January 14/15 each year, although the winter solstice (shortest daylight) occurs around December 21/22.

Sun and his chariot 

According to Puraans, the Sun rides in a chariot guided by Arun, the charioteer, and pulled by seven horses¹. Vishnu Puraan describes the Sun's chariot (2.8.1-7) as 9,000 yojan (vedic unit of measurement) wide and the shaft connecting the yoke to the chariot measuring double that width. It is stated that the bigger axle is 15,700,000 yojan long and the smaller axle is 45,500 yojan long. The wheels represent the cycle of time. The seven horses are called Gayatri, Brhati, Usnik, Jagati, Tristup, Anustup and Pankti, after the seven types of vedic verses. The Sun is described as stationary, although it appears to be rising and setting to people on Earth (Vishnu Puraan 2.8.15).

The auspicious nature of the beginning of the northward journey is mentioned in the epic of Mahabhaarat. The grandfather of Kauravs and Pandavs, Bheesm, could choose the time of his own death. When Bheesm fell during the battle of Kurukshetra, the Sun was going southward. Considering it inauspicious, he waited until the Sun turned northward before relinquishing his life (Mahabhaarat, Bheesm Parv or part 6, chapter 120 and 122).

Notes
¹ Sunlight consists of seven colors and a week also has seven days.