According to Hindu Mythology Ganga descended to the Earth on Ganga Dussehra day. When mighty Ganga was descended to the Earth, Lord Shiva took her in His hairs to break Ganga's descent to avoid Ganga sweeping the whole Earth away. Later Lord Shiva released Ganga so that she can accomplish her mission to purge the cursed souls of Bhagiratha's ancestors.
While on her way to kingdom of Bhagiratha, her mighty flow and gushing turbulent water destroyed the Ashram of sage Jahnu. This angered Sage Jahnu and he drank up all of Ganga's water. Upon this, Bhagiratha and the Gods prayed to Rishi Jahnu to release Ganga so that she could proceed on her mission. Pleased with prayers Jahnu released Ganga from his ear.
According to legends Jahnu Rishi released Ganga from his ear on Saptami of Vaishakha Shukla Paksha. Because of this legend this day is also known as Jahnu Saptami. Ganga is also known as Jahnavi, the daughter of Rishi Jahnu.
On Ganga Saptami devotees worship Goddess Ganga and take bath in Ganges. Taking bath in Ganges on Ganga Saptami day is considered highly auspicious.
This Mythological Story finds mention in Padma Purana, Brahma Purana and Narada Purana.
On this day, people take bath in Ganga river, recite Ganga Stotram and can also offer jal to shivlinga chanting ॐ गंगाधराय नमः॥
