This is literally the "birthday or descent of mother Ganges - Ganga Ma". Throughout india this festival lasts ten days beginning on the Amavasya (dark moon night) and going through to the dasami tithi (tenth phase of the Moon, the day before Pandava Nirjal Ekadasi)
"Festivals connected with rivers are essentially bathing festivals. Ganga Dussehra is celebrated on the tenth day of Jyeshtha. River Ganga is worshipped as a mother as well as a Goddess, particularly by people of Uttara Pradesh, Bihar, and Bengal through which the river flows. On this day, if a devotee is unable to visit and bathe in the river Ganga, then Ganga jal (water) kept in most Hindu homes is used for purification. A bath in the river is said to purify the bather of all sins. The Ganga is revered all over India even in places far from its course.