Onam runs for ten days in the Malayalam month of Chingam, from the asterism Atham to Thiruvonam, and it is kept across Kerala by Hindus, Christians and Muslims alike, which is unusual for a festival with a Puranic story behind it. The reason is that Onam is also the harvest, and the observance is domestic before it is temple based.
The house is prepared for a guest. A pookkalam, a circular carpet of flower petals, is laid in the courtyard on the first day and enlarged each day until Thiruvonam. Small conical clay figures called Thrikkakara Appan are set up and worshipped in the home, named for the temple at Thrikkakara near Kochi that tradition identifies as Bali's seat. The Onasadya, a vegetarian feast of many dishes served on a plantain leaf in a fixed order, is eaten on the last day. Elsewhere there are the boat races of the backwaters, the Athachamayam procession at Tripunithura, tiger dances, swings hung for children, and new clothes given within families.
The framing matters as much as the customs. Onam is not kept as the anniversary of a demon's defeat but as the day a loved king is allowed home to see whether his people are still well and still honest, which is why the greeting and the songs speak of a time when all were equal and there was no deceit. Some modern readings in Kerala press that further and treat Bali as a native ruler displaced by an incoming order, a reading the festival's own affection for him makes easy to argue and hard to settle.