Festival of Mahashivaratri is celebrated on the 14th day of krishna Paksha of the month Falgun (Phalgun- around February-March of Gregorian Calendar). According to sacred scriptures, the devotees who sincerely worship Lord Shiva on this day, are able to please Lord Shiva and this fact is even said to have been declared by Lord Shiva himself, when his consort Parvati asked him as to which ritual performed by his devotees pleases him the most.
Even till date, devotees of Lord Shiva perform the ritual worship of Shivratri with lots of devotion. They observe day and nigh fast and give sacred bath to Shiva Linga with honey, milk, water etc. Hindus consider it extremely auspicious to worship Lord Shiva on a Shivaratri as it is believed that worship of Lord Shiva with devotion and sincerity helps a devotee get rid of past sins.
The date of Mahashivaratri or Shivaratri in 2011 is March 3, 2011. (Thursday)