Festivals link us to our ancestors. Although we do not quite know when this particular festival originated, it is probably fair to say it goes to ancient times. It is a festival that celebrates Kaveri's rich bountiful water, and its rise in level due to monsoon rains in the hilly region that feeds the freshly planted rice seedlings.
Like all festivals, there are local traditions. Their variation would be an interesting to capture in this forum. Perhaps you could describe how your family celebrates it. This will add richness to the discussion.
Let me start off with my wife's description. She is originally from Erode. The celebration is centered around the concept that Kaveri is pregnant... because she is full. So the festivities include pretty much all that we include for a pregnant young woman. Locals in Erode region would go to the river shore, create seven sand kannimar, decorate them and then do a full course poojai. Recently wedded couples who save their wedding garlands to give it to Kaveri on Aadiperukku.
My family lived for a good part outside India. Although I did not pay a great deal of attention to the details when I was young (1950s), I do recall my father saying that it is a celebration of Kaveri's richness. He would have a silver sombu filled with water to represent Kaveri (similar to a Kalasam without a coconut), decorate it with yellow threads, place it in the pooja and do a traditional padayal.
We now follow my father's version in the US for the last 40 years. Would be delighted to hear how you celebrate Aadiperukku.
People like us who are natives of North Arcot and South Arcot celebrate it with preparing variety rice ( which is supposed to be taken to river bank for picnic) and eating that at home.
In Chettinad , where we moved 3 generations back, it is celebrated only by brahmins again with variety rice.
My Mother's place South Arcot district used to celebrate a festival similar to Adi 18 , after Mattu pongal as Atru Thirunal on the banks of Then pennai. It is 3 days after mattu Pongal ( after the 2 karinals).
dear friends: Happy greetings.. our fond hero vandhiyadevan saw cauvery in full spate at kollidam when crossed it in aadiperukku... this year too we see cauvery flooding all through..a good omen for all.. people in cauvery delta zone celebrate it happily and as is our tradition we celebrated it in our home though we were away from cauvery... wish u all a happy time in chennai and elsewhere...