Shivaratri - sometimes written Shivratri - means Night of Shiva. This is a day in honour of the god Shiva and is held on the 13th or 14th day of every month.
But once a year, there is a Maha Shivaratri - or Great Night of Shiva - on the new moon day in Maagha, the 11th month in the Hindu calendar. This usually falls in February or March, before the arrival of spring.
In 2017, the date of the Maha Shivaratri is Friday, February 24.