Bhogi/Bogi festival is celebrated to rid the society of the bad and the evil. The first day, Bhogi, is celebrated on the last day of the month of Margazhi.
Scholars have often compared Bhogi to the Indra Vizha celebrated by the Chola kings at Kaveripattinam, also known as Poompuhar.
Indra Vizha was celebrated in honour of Lord Indra, also called Bhogi, the God of thunder and rain.
Thus Bhogi festival or Bhogi is the first day of Pongal celebrated in honor of Lord Indra, "the God of Clouds and Rains" is also known as Indran (bhogi).
Legends say that on this day that Lord krishna lifted the Gobardhan mountains on his little finger. Lord Indra is worshiped for the abundance of harvest, thereby bringing plenty and prosperity to the land.
The speciality in Bogi or Bhogi festival
People clean up their houses, collect old clothes and unused stuff and burn them. They get a new hope that the harvest festival celebrated on the next day will bring wealth and the good to the society.
The entire house is cleaned and the houses are painted white (usually with lime). All the farm animals are given a big and clean bath. They are decorated with colored powders and paints. All the junk in the houses are either burnt or thrown away.
Reference: Hindu Blog