Sankranti means brilliance. Observing and contemplating that divine splendour on this day is Sankranti.
Every month, the sun moves from one zodiac sign to other, that day is considered as Masa Sankranti. In a calender year, the first 6 months is regarded as Uttarayana and other 6 months is observed as Dakshinayana.
Sankranti is being celebrated because the Sun leaves Dakshinayana, enters Uttarayana into Makara (The Capricorn) zodiac sign and stays there for a month and subsequently enters Kumba (the Aquarius) Meena (The Pisces), Mesha (The Aries), Vrishabha (The Tarus) and Mithuna (The Gemini) zodiac signs.
After his moment in all these zodiac signs, he enters Karkatak (The cancer), which is regarded as Dakshinayana. He stays there for a month and subsequently enters Simha (The Leo), Kanya (The Virgo), Tula (The Libra) Vrishchika (The Scorpio), Dhanu (The Sagittarius) Zodiac signs.
Legend as it, Dakshinayana is sleeping time for deities, and Uttarayana is waking up time.
But the fact is, as per Vedas, all deities are rishis who always rejoice in eternal consciousness. All defeated the sleep (The Tamas). Both are preferred times for worship.
Meditation happens in Dakshinayana. Buddha meditated during dakshinayana. People observe Chaturmasa diksha during Dakshinayana.
Both Dakshinayana, during Kartika and Shravan Masams and Uttarayana (Shivaratri) glistens with the worship of Lord shiva, the lord of lords.