The name of this dish is intriguing because it is neither a spicy (kharam) dish nor has any semblance to the dish called adai. It is a very simple dish prepared with rice and karamani (black eyed beans) on a special festival called Karadaian Nombu. Women perform this vratham (a vow including fasting, praying and then breaking the fast with the offering prepared to the Almighty) for the well being and long life of their respective husbands. Unmarried girls perform the rituals aspiring to get good husbands.
The story of this festival, as per the Puranas, (Hindu religious texts) goes like this. Goddess Parvati covered Lord Shiva's eyes in a playful mood, thus causing the entire universe to come to a stand still. For this shortcoming, Shiva bid her to live on the Earth (Bhooloka) until he decided to take her back.
Goddess Parvati was then born on earth as Kamakshi. She formed a Lingam (symbol of Shiva) with sand, and started her severe penance under a mango tree located at Kanchipuram for her reunion with Lord Shiva. When the spate in the Kamba river threatened to wash away the Sand Lingam, Kamakshi prayed to Shiva offering a dish made of whatever was available - which is the common wild eye peas (also called pigeon peas in US) and rice flour. Lord Shiva then appeared and took her back with him.
Women pray to Kamakshi on Karadian Nombu ,and offer the karadai back to her.
The legend of Savithri and Sathyavan also mentions about this nombu. Here Savithri performs this penance for the Sathyavan's long life. Although his death was preordained, Savithri s is able to save his life from Lord Yama, through her stringent penance. Hence Karadian Nombu is known as Kamakshi Nombu and Savithri Nombu as well.
A sweet karadai can be prepared using jaggery and is known as vella adai. The version prepared with salt and seasoning is called as uppadai. The basic ingredients and the initial preparations are the same for both.