Most festivals have some interesting legends associated with it. Pongal is no different.
Bhogi has an association with Lord Indra (the God of clouds and rains) and Lord Krishna. Earlier, people used to worship Lord Indra who was the King of the deities.
This honour given to Lord Indra made him full of pride and arrogance. He thought himself to be the most powerful.
When Krishna came to know about this, he persuaded his cowherd friends to worship mount Govardhan rather than Lord Indra.
This angered Lord Indra and he sent forth the clouds to generate heavy rains. Meanwhile, Lord Krishna lifted the huge Govardhan mountain on his little finger to protect the cowherds and the cattle. He kept standing with the lifted mountain to save everyone from the ravaging storm. The rains continued for three days and at last Indra realised his mistake. He begged for Krishna’s forgiveness.
Since then, Krishna allowed Bhogi celebrations in honour of Indra.
According to another legend, Lord Shiva once asked his bull Basava to go to earth and deliver his message to the people to have an oil massage and bathe daily, and eat once a month. Accidentally, Basava asks people to have an oil massage and bathe once a month, and eat daily. Enraged, Shiva cursed Basava and said that due to this mistake there would be lack of grains. He banished the bull to live on earth forever and help people plough the fields. Thus, Mattu Pongal has an association with cattle.