This day is auspicious because on this day the sacred Ganges descended on the earth. It can be called the birthday of Ganga. A dip in the Ganges or in the other river invoking Goddess Ganges, is beneficial and is said to be purified from ten sorts of sins. Worship of the river deity is done by incense, light, sandal wood, flowers, milk, etc.. Flour balls are fed to aquatic animals.
There was a ruler named Sagar. He performed Ashva Medha Yageya. God Indra stole away the ashva (Horse). Sagar "s grandson Anshuman took over the responsibility of the search of the horse. After searching all over, he reached Netherlands with his 60,000 followers, where he saw god personified as Kapil saint, was under meditation and the horse was grazing nearby.
The followers of Anshuman shouted, "thief-thief". Thereupon Kapil rishi opened his eyes and by the blazing lightning of which all were reduced to ashes. For the final cremation ritual of those dead, Bhagirath undertook severe austerities. Brahma when pleased and asked him to demand a boon. Bhagirath requested for the descent of Ganges on land. Brahma agreed but expressed his apprehension if the earth could shoulder the weight and flow of the Ganges, in that case of Bhagirath should invoke God Siva. When Siva agreed to hold the fall of Ganges, Brahma let it loose from the heavens (swarga lok).
The Ganges was then interlocked in the tufts of Siva and Bhagirath's purpose, of washing the ashes of his ancestors in the water of the Ganges, remained still. There upon Bhagirath invoked Siva, who when please, released the Ganges as river Ganges on the land. The release of the sacred water of the Ganges and its flow through different cities of India is very fortunate, pious, auspicious for the Hindus of India.
Ganges was not merely a river. She (Goddess Ganga) was devoted to the service of lord Krishna in the heavens (Swarga-Baikuutth) She was thus very near the lord, which made Radha jealous. The latter cursed her to go down to earth and flow as a river. Ganga, in retaliation, also cursed Radha that you will be close to Krishna, yet you will always imagine him far away, tolerate separatism and never be peaceful. Thus since then Ganga flows as a Ganges river, under the ancient curse of Radha and under the modern curse of city pollution. In the modern times of Kaliyuga, Ganges is of utmost and importance, as according to Narada Purana, all pilgrimages were of influence in Sateya yuga then Pushkar in Treta and Kurukshtra in Dwapar Yuga.