According to the hindu Mythology, Lord shiva once asked Basava (Bull) to visit on the Earth and ask the Human to have an oil massage and bath every day. But Basava (Bull) announced that eat daily and have an oil bath once in a month. This makes the Lord shiva furious and he cursed the Basava (Bull) to live on the Earth forever and said that Basava (Bull) have to plough the fields and help people produce more food. Hence, people after harvesting celebrate this festival with crops and cattle.
It is basically harvesting festival or it can considered as the 'thanks giving festival' because this festival is celebrated to thank the sun god and Lord indra for helping farmers in getting better-yielding crops. During the festival people reject old belongings and welcome new stuff.
As we know that india is an agricultural country and the majority of the festivals are inclined towards nature. Just like another festival, the pongal is referred to as Uttarayan Punyakalam which bears special significance in hindu mythology and is considered extremely auspicious.