Karka Sankranti marks the Sun's entry into Karka, the fourth sign of the sidereal zodiac, corresponding to Cancer. It is fixed by the Sun reaching a longitude of ninety degrees against the fixed stars, computed with the Lahiri ayanamsa, and usually falls in mid-July.
From this point the Sun begins Dakshinayana, the half of the year in which its rising point moves southward and the days shorten, the counterpart of the northward Uttarayana that starts at Makara Sankranti. This is a purely solar marker, defined by the Sun's ecliptic longitude rather than by a lunar day, and its date in local time can shift by a day between cities.
The six months of Dakshinayana are traditionally regarded as a period for inner practice rather than new undertakings. In the Tamil calendar the day opens the month of Aadi, and in the Himalayan regions it is kept with fairs and offerings to local deities.