Mithuna Sankranti marks the Sun's passage into Mithuna, the third sign of the sidereal zodiac, corresponding to Gemini. It is fixed by the Sun reaching a longitude of sixty degrees against the fixed stars, calculated with the Lahiri ayanamsa, and generally falls in mid-June.
This is a solar reckoning, tied only to the Sun's place on the ecliptic and not to any tithi or lunar phase, which sets it apart from the lunar-month festivals. Because the transit instant is expressed in local time, the observed date and the timing of associated bathing or giving can vary by a day between cities.
In Odisha the day begins Raja Parba, a three-day festival that treats the earth as menstruating and about to be renewed by the monsoon, during which farming is paused. In Assam it aligns with Ambubachi at the Kamakhya temple, and in the Tamil calendar it opens the month of Aani.