Simha Sankranti marks the Sun's passage into Simha, the fifth sign of the sidereal zodiac, corresponding to Leo. It is determined by the Sun reaching a longitude of one hundred and twenty degrees against the fixed stars, using the Lahiri ayanamsa, and normally falls in mid-August.
As with every sankranti this is a solar event, set by the Sun's position on the ecliptic and not by any tithi or phase of the Moon, which is what separates it from the lunar-month festivals. The transit instant is converted to local time, so the observed date and the timing of ritual bathing or giving can differ by a day from one city to the next.
In the Tamil calendar the day opens the month of Aavani. In parts of the Himalayan belt it is observed as Sair or Sazza with the exchange of seasonal produce, and in Kerala it falls within the Onam season and is marked by temple offerings.