It is stated in the Kārttika-māhātmya section of the Padma Purāṇa:
śuklāṣṭamī kārttike tu
smṛtā gopāṣṭamī budhaiḥ
tad-dinād vāsudevo ‘bhūd
gopaḥ pūrvaṁ tu vatsapaḥ
“The eighth lunar day of the bright fortnight of the month of Kārttika is known by authorities as Gopāṣṭamī. From that day, Lord Vāsudeva served as a cowherd, whereas previously He had tended the calves.”
The word padaiḥ indicates that Lord Kṛṣṇa blessed the earth by walking on her surface with His lotus feet. The Lord wore no shoes
