Yadaa yadaa hi dharmasya glanir bhavati Bharat
Dharma sansthaapanaarthaaya sambhaavami yuge yuge

i.e. I incarnate whenever Dharma declines and evil predominates, to establish Dharma and vanquish evil.
In the Bhavishyottara Puran Shri Krishna himself extols the celebration of Janmashtami to Yudhishtir, the eldest of the five Pandavas. He incarnated in Mathura (Uttar Pradesh) to vanquish the evil King Kansa and other evil elements who were harassing pious people. His second reason was to fulfil the pledge he had given to Vasudev and Devki. When they married Kansa heard a divine proclamation that the couple's eighth son would be his nemesis. To obviate this, Kansa imprisoned the couple and killed the first seven sons born to Devki. When Shri Krishna was born Vasudev placed him in a cradle and took the baby child through the river Yamuna. Thus the Lord survived and eventually vanquished Kansa and other evil elements.                 

The Ceremony of Dahi-Handi

During this ceremony a large earthenware pot is filled with milk, curds, butter, honey fruits etc. and is suspended from a height between 20 to 40 feet. Sporting young men and boys come forward to claim this prize. To do so they construct a human pyramid by standing over each other's shoulders till the pyramid is tall enough to enable the topmost person to reach the pot and claim the contents after breaking it. Normally, currency notes are tied to the rope by which the pot is suspended. This prize money is distributed among those who participate in the pyramid building.

This ceremony replicates Krishna's love for milk and butter. In his childhood, the Lord Krishna alongwith his mates used to raid the houses of his neighbours in search of milk and butter.                                    

Festivals & Rituals

People fast on this day, usually a waterless fast. 
They spend the day immersed in Shri Krishna's glory by reading, reciting and singing his divine leela especially in the evening in mandirs. 
At midnight, the Lord's birth hour, arti is performed. He is also installed in the form of 'Lalji' (child form) in a swing and devotionally offered many sumptuous food dishes. 'Makhan' (butter) is especially included since Shri Krishna loved this in childhood. The traditional prasad is 'Panchajiri' - made of five ingredients: powdered ginger, 'suva', coriander, sugar and ghee. Other ingredients include poppy seeds (khaskhas) and dessicated coconut shavings. After arti, devotees jubilantly sing and hail kirtans extolling the Lord's incarnation.