The puja performed on the fourth day of Diwali is called the Govardhan Puja. The origin of this day goes back to the Dwapara Yuga, and to Lord Krishna. According to legends, he lifted mount Govardhan on this day and hence the day is dedicated to the worshiping the mountain. Goverdhan puja is offered as a tribute to Krishna's heroic feat. In parts of north India, people make cow dung replicas of the fabled mound, decorate it with flower petals and offer prayers.
The people of Mathura, where Krishna lived, were basically farmers and had a practice of offering yearly puja's to Lord Indra. The worship was a means of thanking for a good produce and all the celebrations happened at mount Govardhan, near Braj. One particular year, as usual the residents of Mathura started their journey towards the mountain and Krishna accompanied them.
People were preparing to carry out the pooja to please Indra, when Lord Krishna approached them and questioned their belief practice. He told them to worship the fields where they grow their produce and the cattle which are help in planting the crop, instead of Lord Indra. The elders in the community said that it would displease Lord Indra and bring disaster. But, Krishna was persuasive and they yielded to his argument.
The people of Mathura under the guidance of Krishna made all the preparations for worshiping the cattle and the fields, when terror struck. Lord Indra angered by the insult of created a storm which threatened to blow away not only the cattle but also the people. The scared people ran to Krishna for help and he lifted the mountain Govardhan with his little finger. The people and the cattle took shelter under the mountain and were saved from the storm. Humbled by this act, Lord Indra, approached Lord Krishna to apologize.