Jahnavi meaning - water Ganga that came from rushiking jahnu - in the month of vaisaka on saptami day.

 Seeing this, the devas prayed to the sage to release Ganga, so that she could proceed on her mission to release the souls of the ancestors of Bhagiratha. Jahnu relented, and he released the Ganges from his ear.^[4] For this, the Ganges river is also known as Jahnavi, meaning "daughter of Jahnu"

https://en.wikipedia.org/wiki/Jahnu

Seventh day in Vaisakha maasa (Sapthami) is known as Ganga Sapthami or Gangotpatti the day Holy Ganga came out of Sage Jahnu’s ear and thenceforth came to be known as JaAhnavi

https://archive.org/details/niceties-of-sanatana-maasa-dharma/page/14/mode/1up