Today, February 12, 2010, is the 14th day of Krishna Paksha in Phalgun month is the night of lord Shiva – Mahashivratri. The corresponding period in many regional calendars is the 14th day of Krishna Paksha in Magh month. The entire day and night on Mahashivarathri is dedicated to Lord Shiva. Observances include fasting and keeping vigil at night. Special pujas are performed at midnight.
You can read more about Mahashivratri in this article