Masi Magam
Tamil month masi occurs during Feb-Mar every year. During this month, when the nakshatram (meaning star) named magam (or makha / magha) falls on pournami (poornima / full moon day), it is celebrated as Masi magam. Even in the years when the nakshatram magam does not prevail during pournami, masai magam is celebrated. This festival is specially celebrated in Tamilnadu.
Celebration in temples:
Deity from temples (utsava moorthy) are taken for procession for theerthavari - is dip in the river or any other theertham; it could be pond or river or sea. Devotees walk along with the deity during the procession and they take a dip too, to remove all the sins.
This masi magam festival is even more special in Kumbakonam. The festival is celebrated in a grand manner in all the temples in kumbakonam. People from surrounding places visit kumbakonam and take dip in the river cauvery (or kaveri). Generally people take dip in holy rivers to get rid of the sins. It is believed that gods of all the holy rivers, like ganga, yamuna, saraswati, take a dip in kaveri on this day to get rid of the sins accumulated in them that was let out by people.
Mahamaham:
Masi magam is celebrated as mahamaham is celebrated once in 12 years, when sun is in kumbha rasi. Maha maham day is the concurrence of 3 events - month of magam, full moon day, sun passing through kumbha rasi.
It is believed that during the maha pralayam, siva saved necessary seeds and amritam along with other items in a pot containing water. When the maha pralayam came to an end, the pot landed in Kumbakonam. Siva came as a hunter and threw an arrow in the pot, the water pot then became a pool which is now referred as Mahamaha kulam (mahamaha tank).