On this day lord Sri Krishna became a qualified cowherd. Before this day, he was a keeper of the calves.
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."
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.
Krsna said at that time that the cows are worshiped even by the demigods, and He practically demonstrated how to protect the cows. At least people who are in Krsna consciousness should follow in His footsteps and give all protection to the cows. Cows are worshiped not only by the demigods. Krsna Himself worshiped the cows on several occasions, especially on the days of Gopastami and Govardhana-puja.