Onam is one of the most traditional and popular festival which is celebrated in the state of Kerala. It is also called as harvest festival of Malayalam. People believe that the spirit of Mahabali visit Kerala during this festival. Every year this festival held on August and September the festivals go on for a period of ten days each day there is different celebrations, this year celebration starts from August 22nd to September 2nd. It's already started in 2020.
And to add to what Sriluuppari explained, the one interesting feature is that Onam is celebrated by people of all religions in Kerala. Its more a cultural fest than a religious one, so its a state-wide celebration