Vrishchika Sankranti marks the Sun's entry into Vrishchika, the eighth sign of the sidereal zodiac, corresponding to Scorpio. It is fixed by the Sun reaching a longitude of two hundred and ten degrees against the fixed stars, computed with the Lahiri ayanamsa, and usually falls in mid-November.
This is a solar reckoning, tied only to the Sun's position on the ecliptic and not to any lunar day or phase, which is what sets it apart from festivals fixed to the lunar month. The transit instant is expressed in local time, so the observed date can shift by a day from one city to another.
In the Tamil and Malayalam calendars the day opens the month of Karthigai and Vrishchikam. It begins the forty-one-day observance kept by pilgrims to Sabarimala in Kerala, and in Tamil Nadu it leads into Karthigai Deepam, the festival of lamps held later in the month.