Sankranti literally means movement. It is the movement of Sun from one Rashi (zodiac) to the other. Vishnupadi Sankranti is the movement of Sun (Surya) to Vrishaba, Simha, Vrischika and Kumbha Rashi. The entry of Sun is also known as Vishnupadi Punyakalam.
Vrishabha Sankranti (May 14 or 15) – Transit of sun
from Mesha or Meda Rashi to Vrishabha Rashi (Aries to Taurus). Prayers are
offered to Vishnu and Shiva on the day. Donations are made. People also perform
holy dip in sacred rivers.
Simha Sankranti (August 16, 17 or 18) – Transit of sun
from Karka Rashi to Simha Rashi (Cancer Zodiac to Leo Zodiac) – Narasimha,
Vishnu, Shiva and Surya are worshipped. Ganesha is offered special pujas and
bath with coconut water. Havan is performed in the name of the various avatars
of Bhagavan Vishnu.
Vrischika Sankranti (November 16, 17 or 18) – Transit
of sun from Tula Rashi to Vrischika Rashi (Libra to Scorpio). The famous
Sabarimala Pilgrimage begins on this day. Vishnu Sahasranamam and Aditya
Hridayam are chanted on the day.
Kumbha Sankranti (February 12, 13 or 14) – Transit of
sun from Makar Rashi to Kumbha Rashi (Capricorn to Aquarius). Devotees residing
along the banks of holy rivers and in most eastern part of India celebrate this
by taking holy dip in Ganges. Offerings are given to cows and to poor people.