Dakshinayana is the southern transit of the Sun. In Hinduism, a human year is equal to one day of gods. Dakshinayana is the night of the Devas and Uttarayana is the daytime. The Uttarayana and Dakshinayana period is calculated differently in South and
North India. In North Indian calendars, Dakshinayana in 2015 begins on June 21. In South Indian calendars, it begins on July 16, 2015.
In South India, Uttarayana period starts with Makar Sankranti on January 14 or 15 and ends on July 14 or 15. Dakshinayana is from July 14 or 15 to Makar Sankranti. This method of calculation of the period of Sun’s transit is known as the Nirayana System and in it Dakshinayana starts when ‘nirayana surya’ enters cancer.
North India, Uttarayana period starts on December 21 and ends on June 21. Dakshinayana is from June 21 to December 21. This method of calculation of the period of Sun’s transit is known as Drika Siddhanta and in this method Dakshinayana starts when sayana Sun enters Cancer.
The Chatur Mas or Four holy months in Hinduism falls during the period of Dakshinayana.