It’s also known as Indrotsava. It began in the time of the Chandravanshi king Vasu. The king prayed to Indra for a long time and was gifted a Venudanda. He planted this pole on the ground and conducted an Indrapuja. From then on, this is the ritual followed for Indrapuja. The pole should also be decorated flowers and sandalwood garlands. The idol of a swan is also fixed at the top of the pole, because Indra visited Vasu in the form of a swan, as per Mahabharata Adiparva Chapter 63.