Kark Sankranti is the summer solstice of Sun and it marks the beginning of the Dakshinayana Period in traditional Hindu Solar calendars. The term Sankranti in a Solar Calendar generally refers to the means the first day of each month – when the sun passes from one constellation to another. In 2015, the date of Kark Sankranti is July 17 in South Indian calendars. In North Indian calendars Kark Sankranti 2015 begins on June 21.
Kark Sankranti is the southern transit of the Sun and it ends with the Uttarayan or Makar Sankranti in mid-January. The nights are longer during Kark Sankranti Period.
Kark Sankranti marks the beginning of the night of Devas for some Hindu communities. There is also a popular belief that Lord Vishnu goes to sleep during this period and the Ekadasi just before Karka Sankranti is referred as Dev Sayani Ekadashi.
In Tamil Nadu, the month Aadi marks the beginning of Kark Sankranti.
It must be noted that Kark Sankranti or Dakshinayana is calculated differently in North Indian Hindu calendars and other regional calendars.