Akin to Kumbh Melain North India, Mahamagamis  religiously famous in South India.  This  takes place once in twelve years, in Kumbakonam in Tanjore District, an  old town with innumerable famous temples, in the Tamil month of Masion Full Moon Day when  the star is Magamon  that day.  This year, it falls on March 6.
It is believed that on this day,  all the nine sacred Rivers of Bharat converge in the nine wells in the Temple  Tank known as Mahamagam Tank and it is believed to be sacred for anyone to take  a bath in the Tank on that day to ward off evil influences. 
The Temple Tank  known as Mahamagam Tank is  12 Acres in area and has well-laid out steps on all the four sides.  In  the centre of the tank, there are 9 wells in which the Holy Rivers  converge.  On the four banks of the Tank, there are 16 Sannathis for 16 Mahalinga Swamis, known as Shodasa  Lingas.  (These 16 temples are said to have been built  by one Sri Govinda Deekshitar, who was the Chief Minister for the King,  Achuthappa Naicker around 1545 A.D.  The King gave him gold by his weight,  with which he has built these temples.)
Over 50 lakhs pilgrims are  expected to come for this Mahamagam festival and take a holy dip in the  Tank.  About 20,000 police personnel have been mobilized to take care of  security with tension having increased in the Police circle on account of  the expected visit of the Chief Minister, Jayalalithaa. 
When Lord Guru (Venus) enters Leo  on February 26, the tanks get the sanctity and hence, though Mahamagam falls on  March 6, a dip in the tank on any day between February 26 and March 6 is  considered sacred.  Dhwajarohanam (Flag hoisting) was done on February  26  
It is learnt from an Inscription  that King Krishna Devaraya took bath in this temple on 24th February 1518 and  distributed lot of presents to all.  From this it will be clear that  Mahamagam was celebrated even in 1518 A.D. and this should have been in  observance from time immemorial.
Article by ‘Media   Center, Chennai’ 
Another article on Festival by Laura Kelly
  It is just the Kumbh Mela of the South   India. Kumbha Konam a town near Thanjavur derives it names from  'Kumbh' or pot which settled after the great 'pralaya' that covered the earth  with water. The pot contained seeds of all life forms, hence Kumbakonam.
  Brahma was advised by Lord Shiva to put the Germs of all creation into a pot or  vessel caped with a coconut, and save it. When the Pralaya destroyed the earth  and covered it with water, the pot alone survived and settled that later became  Kumbha Konam. The festival occurs once in 12 year, when Maha star, falls on  Simha rasi. All the sacred rivers of India are said to come here during the  auspicious hours and cleanse themselves. One can judge the age of the festival,  by the fact, that it mentions the long bygone rivers like Saraswati and Kumari.  Saraswati was in the North while Kumari river was beyond the present  Kanyakumari i.e. in the Indian Ocean, when that part of India was still intact  and not swallowed by the sea. 
Also see 
  1. Visit www.kumbakonam.com 
  2. Temples of Kumbakonam 
  3. Seven Sacred Rivers