On Gopastami, Lord Krishna became a skilled cowherd. Previously, He used to handle the calves only. In the month of Kartika, the 8th lunar day is considered as the Gopastami by the associate authorities.
When boys stepped into the age of 6 years, they are called pauganda and that time they also got the charge of handling the cows as well. This was the day Nanda Maharaja sent his children Krishna and Balarama to herd the cows for the first time. It was a celebration meant only for men and boys. As Srimati Radharani and Her friends wanted to enjoy the fun, and because of Her resemblance to Subala-sakha, She puts on his dhoti and garments and joined Krishna. The other gopis joined in too.
Devotees remember this sweet pastime on Gopashtami day.