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." (see the entire pastime by clicking here)
In His honour, and thinking of Him one should worship the cows with ten upacharas, with the mantra "om gave namah" etc. In some places they dress a small boy as Krishna to invoke stronger remembrance of the pastime.
One can anoint or "paint" the horns with turmeric and put sindura on their foreheads, and hand marks on to their bodies. One should then feed them fresh sweet grasses and circumambulate them.
See simple prayog for Go-pujaom namo gobhyah srimatibhyah saurabheyibhya eva ca
namo brahma sutabhyas ca pavitrabhyo namo namah
Other events today:
Sri Gadadhara Das Goswami - Disappearance
Sri Dhananjaya Pandit - Disappearance
Srinivasa Acarya - Disappearance