Onam started with Athachamayam procession."Atham pathinu ponnonam". This is a statement famous among Keralites, which means Onam is on the tenth day after Atham. Today is the asteric Atham in the month Chingam. Hence on tenth day from today is the famous Onam (asteric Thiruvonam). Today, this day is celebrated at Thripunithura, the suburban town of Kochi. That town was the Capital of ex-Kochi Raja. The function there is known as "Atha chamayam". A procession through the streets of that town is a picturesque one. Lots of people dressed in different forms, caparisoned elephants, horse riders, different classical dance forms, kathakali forms and several others join the procession. The whole town will have short term traders with their products exhibited.
Actually there is a history behind this. It is a reminder of a procession starting from Kochi Raja's head quarters to Thrikkakara where the Lord Vishnu's temple is situated and the festival at the temple celebrations start today.