Kumbha Sankranti marks the Sun's entry into Kumbha, the eleventh sign of the sidereal zodiac, corresponding to Aquarius. It is fixed by the Sun reaching a longitude of three hundred degrees against the fixed stars, computed with the Lahiri ayanamsa, and usually falls in mid-February.
This is a solar reckoning, turning only on the Sun's position along the ecliptic and not on a tithi or the Moon's phase, which distinguishes it from the lunar-month festivals. Because the transit instant is stated in local time, the observed date can move by a day between cities, and the customary bath and gift are aligned to the nearest sunrise.
The day is observed along the Ganga and other rivers with ritual bathing and donations to those in need, and it is one of the sankrantis noted in the almanacs of the north and east. In the Tamil calendar it opens the month of Maasi.