Gopashtami day is a famous festival in Mathura, Vrindavan and other Braj areas, which is observed on Ashtami Tithi of Kartik Shukla Paksha.
As per Hindu mythology, Lord Krishna lifted Gowardhan Mountain on His smaller finger on the day of Gowardhan Puja to save Braj people from the fury of Indra. The story goes like this. Lord Krishna advised the people of Braj to stop annual offering made to God Indra. This made Indra angry and he started flooding Braj region. However, God Indra failed in his mission because Braj people and their livestock were protected under the vast shelter of Gowardhan hill lifted by Lord Krishna with his small finger. After severe flooding that continued for seven days in Braj region, God Indra accepted his defeat on Gopashtami.
Gopashtami is the day when cows and their calf are decorated and worshipped. This ritual of worshipping cows and calf is similar to the Govatsa Dwadashi ritual performed in Maharashtra.
Mantra to Chant While worshipping cow:
लक्ष्मीर्या लोकपालानां धेनुरूपेण संस्थिता।
घृतं वहति यज्ञार्थ मम पापं व्यपोहतु।।