Gopastami
On this day Lord Sri Krishna became a qualified cowherd. Before this day, he was a keeper of the calves.
This event is described in Srimad-Bhagavatam (10.15.1):
tatas ca pauganda-vayah-sritau vraje
babhuvatus tau pasu-pala-sammatau
gas carayantau sakhibhih samam padair
vrndavanam punyam ativa cakratuh
“When Lord Rama and Lord Krsna attained the age of pauganda [six to ten] while living in Vrndavana, the cowherd men allowed Them to take up the task of tending the cows. Engaging thus in the company of Their friends, the two boys rendered the land of Vrndavana most auspicious by imprinting upon it the marks of Their lotus feet.”
As stated in the purport: “Since Lord Krsna’s spiritual body had apparently grown slightly in age and strength, the senior men of Vrndavana, headed by Nanda Maharaja, decided to promote Krsna from the task of herding calves to the status of a regular cowherd boy. He would now take care of the full-grown cows, bulls, and oxen. Out of great affection, Nanda Maharaja had previously considered Krsna too small and immature to take care of full-grown cows and bulls. It is stated in the Karttika-mahatmya section of the Padma Purana:
suklastami karttike tu
smrta gopastami budhaih
tad-dinad vasudevo ’bhud
gopah purvam tu vatsapah
‘The eighth lunar day of the bright fortnight of the month of Karttika is known by authorities as Gopastami. From that day, Lord Vasudeva served as a cowherd, whereas previously He had tended the calves.’
“The word padaih indicates that Lord Krsna blessed the earth by walking on her surface with His lotus feet. The Lord wore no shoes or other footgear but walked barefoot in the forest, giving great anxiety to the girls of Vrndavana, who feared that His soft lotus feet would be injured.”
The celebration was meant for only the cowherd men and boys, but Srimati Radharani also wanted to enjoy the fun, and so, because of Her resemblance to Subala-sakha, she donned his dhoti and other garments joined Krishna. Thus, on this occasion, in temples in Vrindavan and elsewhere, Srimati Radharani is dressed as a cowherd boy.
The Killing of Dhenukasura:
“Thus Sri Krishna, along with His elder brother Balarama, passed the childhood age known as kaumara and stepped into the age of pauganda, from the sixth year up to the tenth. At that time, all the cowherd men conferred and agreed to give those boys who had passed their fifth-year charge of the cows in the pasturing ground. Given charge of the cows, Krishna and Balarama traversed Vrindavana, purifying the land with Their lotus footprints.”