Aadi Perukku, celebrated on the 18th day of the Tamil month of Aadi, is a monsoon festival of Tamil Nadu. Known as Pathinettam Perukku(Perukku means to grow) or Aadi 18, the festival welcomes the new water from monsoon rains in rivers. It is mainly celebrated by Hindus living on and around the banks of Cauvery and its tributaries.

As the monsoon rains bring abundant water to the rivers, frothing and touching the banks, farmers welcome and worship Ma Cauvery by offering flowers, fruits and other auspicious things. They thank the river for giving a good harvest the previous year and pray for a better harvest in the coming year. Farmers start sowing seeds on this day as it is believed that they will grow in abundance.

It is also a festival of women as females of all ages come together to pray to Ma Cauvery to bless them with prosperity. Ma Parvati is worshipped in the form of Ma Cauvery. Married women seek her blessings to beget offspring; the unmarried ones seek her blessings for a good husband; the newlyweds pray to her that their husbands should be hale and healthy and as happy and joyful as Ma Cauvery when she brings the new flood.

For the newlyweds, the first Aadi Perukku is a special occasion as the mangalya thread tied on the day of marriage is replaced with a new one with add-ons like gold coins and mango-shaped gold trinkets, which are considered auspicious. Married women also change the old mangalya thread and wear a new one on this day. Families throng the banks of Cauvery and other waterbodies in the neighborhood to pay their gratitude to nature for blessing them with water and food. Women make a lamp out of rice and jaggery, place it on plantain leaves and let it float on the river.

A variety of rice dishes called Kalavai Sadham, curd rice, lemon rice, tomato rice, sweet pongal, etc are prepared and families enjoy eating them on the banks of the river as they have fun. Children make small chariots and take them on a procession. While Dhanteras is considered as the auspicious day to buy gold and other precious metals up North, Aadi Perukku is considered the most auspicious day to buy gold in Tamil Nadu.