Mahaamaham festival - Kumbakonam, Tamilnadu

Mahaamaham festival is celebrated in Kumbakonam, when Guru Bhagavan is in Simha Raasi, when Pournami or full moon day in the Tamil month of Maasi coincide with Maha natchathiram (star).
It is beleived that
the nine sacred rivers Ganga, Yamuna, Godavari, Narmada, Saraswati, Cauvery, Kumari, Paalaaaru, Sarayu,
the theerthams of the eight directions, Brahma Theertham & the Naaga Theertham
flow from the nineteen wells in the Mahaamaham tank and the 66 crore theerthams of the world into the central Kanniyaa Theertham on this day.

Hence a dip in the Mahaamaha Theertham on this day, rids devotees of all sins. There is also a belief that all the theerthams reach the Mahaamaham tank in order to rid themselves of the sins which the devotees have washed off in their waters.

The story of the formation of Kumbakonam
The tank structure, 20 theerthams, the 16 mandapams & 16 Siva Lingams
Govinda Dikshitar's contribution
The theerthavaari & holy dp in the Mahaamaham tank on 6 March, 2004