Dakshinayanam (Pitrayana) is also referred as Karka Sankraman or Karkataka Sankranti. On this day, Sun enters into Karkataka (Karka) rashi (Cancer). In 2015, Dakshinayana starts on July 17 as per the South Indian Panchangams. According to North Indian calendars, Dakshinayana punya kaal starts on June 21, 2015.
Karka Sankranthi or Karkataka Sankranti indicates ending of the Uttarayan Punya kaal and the beginning of Dakshinayana punya kaal, whereas Makara Sankranthi (Pongal) marks the beginning of Uttarayan Punya kaal.
Dakshinayana or Karka Sankraman – The time to observe Deva Shayana Vrat:
In Hinduism, it is believed that Gods go for sleep during the period of Dakshinayan. The beginning day of Vishnu Shayana Vrata or Chaturmas Vrat is called as Vishnu or Deva Sayani Ekadasi, and the Shiva Sayana Vrat is called as Shiva Shayana Chaturdashi or Pishacha Chaturdasi. During the Karka Sankranthi, performing Pitru tharpan or the rituals to pay our respect to Pithru Devatas is considered as highly meritorious deed.
Performing food charity and donating clothes is also highly meritorious. Worshipping Lord Vishnu, chanting Vishnu Sahasra nama stotram and worshipping Lord Varaha Swamy (the third incarnation of Lord Vishnu) is preferable ritual on the day of Karka Sankranti.
As per Hinduism, for Gods one day equals to one year in Human life. Uttarayan is the day time to Gods and the Dakshinayan is the night time.