It is the great night of Siva which commemorates the night, when Lord Siva
consumed the deadly poison Kalakudam to save the world from destruction.
Sivarathri is an auspicious night observed on the fourteenth night of dark half
of the moon.
Maha Sivarathri : Maha = great; rathri = night. Sivarathri means night of Siva
and Maha Sivarathri means the great night of Siva. Just like there are 24 or 25
Ekadasis, which occur each year, 24 or 25 Sivarathris occur each year. And just
like only one of these Ekadasis is the very special Vaikunta Ekadasi, only one
of the Sivarathris is known as Maha Sivarathri.
It is celebrated in the Malayalam month of Kumbam (February-March). The Annual
Sivarathri Festival held on the banks of Periyar at Aluva, is one of the most
popular and spectacular local festivals of Kerala, which attracts thousands of
people from all over the country.
Find more information about Sivarathri from these sites:
Mahasivarathri - http://www.srisathyasai.org.in