Onam is a national festival and a harvest festival which is similar to baisaki in the north. Onam begins from the day of Atham. From Atham day to thiruonam day people adorn their houses with flowers. They put flowers in a design attractively.
Every festival has a story behind it.
Same way onam also has a story behind it. He was very kind to people. Mahabhali ruled the people very well. As always the devas sought Lord Maha Vishnu's help. So Lord Vishnu donned the disguise of Vamana,a short, bald brahmin.