Ganga Dussehra is an annual celebration of one of the holiest rivers in Hinduism, Ganga aka Ganges River. The celebration lasts for 10 days from the new moon at the beginning of Jyaistha (amanta reckoning). The last day, 10 Jyaistha, is the holiest of them all (this normally occurs in June in the Gregorian calendar, or occasionally at the end of May). Devotees are expected to bathe in the Ganga. If they cannot reach the banks of the river, many devotees will use water drawn from the Ganga that is kept at home to bathe. Alternatively, the devotee will bathe in ordinary water whilst invoking Ganga by prayer. This bathing is considered to impart purity from sin. In 2015, Ganga Dussehra falls on May 27th.
The day is Ganga Dussehra, which is also the day when ages long hard work and penance of Bhagirath, a Suryavanshi king, got successful and fruitful in the form of river Ganga. It was he only whose hard penance was responsible for bringing river Ganga on earth that made a ruined and dry waste region developed into green and fertile and brought back life to earth. Since then every year, the occasion of Ganga Dussehra is celebrated by performing several rites and rituals of Ganga pooja to commemorate her.
When river Ganga incarnated on earth that occasion was obliged with rare ten vedic astrologic calculations. Jyeshtha month, Shukla Paksha (bright half), Tenth date, Wednesday, Hasta Nakshatra, Vyatipata yoga, Gar Anand Yog and Moon in Virgo and Sun in Tauras, these all ten Yogas absorb all the ten sins by merely take a bath in river Ganga on Ganga Dussehra.
Lord Shiva is the prime deity to be worship during the Ganga Pooja as he is sole owner and holder of river Ganga and by grace of his mercy only sent the river on earth for the welfare of mankind. The donation of ten eatable items mainly fruits and black sesame seeds are considered most auspicious.