Ganga Dussehra falls during Dashami Tithi of Jyeshtha Shukla Paksha and currently falls in month of May or June. Ganga Dussehra is also known as Gangavataran which means the descent of the Ganga.
Usually Ganga Dusshra is celebrated one day before Nirjala Ekadashi but in some years Ganga Dussehra and Nirjala Ekadashi might fall on the same day.
Ganga Dussehra is dedicated to Goddess Ganga and this day is commemorated as the day when Ganga was descended to the Earth to accomplish her mission to purge the cursed souls of Bhagiratha's ancestors. Before coming to the Earth, the Goddess Ganga was residing in Kamandal of Lord Brahma and along with her the Goddess Ganga brought the purity of heaven to the Earth.
On Ganga Dussehra devotees worship Goddess Ganga and take bath in Ganges. Taking bath in Ganges and offering charity or Dan-Punya (दान-पुण्य) on Ganga Dussehra day is considered highly auspicious. It is believed that holy dip in Ganges on Ganga Dussehra day can purge all type of sins.
Devotees flock to Prayagraj/Allahabad, Garhmukteshwar, Haridwar, Rishikesh and Varanasi to take a holy dip. Ganga Dussehra celebrations are legendary in Varanasi. On Ganga Dussehra day thousands of devotees do Ganga Snan and participate in Ganga Aarti at Dasaswamedh Ghat.
Ganga Dussehra should not be confused with Ganga Jayanti when the Goddess Ganga was reborn.