The holiday is always celebrated on 21 February. Known as Language Movement Day, Martyrs' Day and 'Shôhid Dibôs' in Bengali, this holiday commemorates the struggle for the Bengali language in 1952.
In 1947, India was partitioned by the British, creating the 'Dominion of Pakistan', which was two separate regions to the north west and north east of India.
Even though the majority of people lived in the eastern part, where Bengali was the main language, the dominion was in the control of the western part. In 1947, the western based government had proposed Urdu as the only state language, and that it would be used exclusively in schools and in the media. This move caused unrest and protests in East Pakistan.
In early 1952, the protests had intensified and the government imposed a law (Section 144), which banned any gathering of more than three people.
On 21 February 1952, In defiance of the law, students began gathering on the University of Dhaka. The police enforced section 144 and arrested several protestors. This further enraged the crowd and when the students attempted to enter the building of the East Bengal Legislative Assembly, the police opened fire and shot dead four protestors.
As a result of the protests, Bengali was recognised as the second official language of Pakistan on 29 February 1956, and the constitution of Pakistan was reworded to "The state language of Pakistan shall be Urdu and Bengali."
East Pakistan gained independence from Pakistan in 1971, becoming Bangladesh.
To mark the language movement and the massacre, a symbolic sculpture, Shaheed Minar (Martyrs' Monument), was erected in the place of the massacre. On 21 February, the area in front of the sculpture is covered in flowers, left as a commemoration of those who gave their lives in defence of their language and culture.
The efforts of the Bangladeshi people to protect their language is honoured by UNESCO which established 21 February as International Mother Language Day in 2000. The intention of the day is to promote awareness of linguistic and cultural diversity and multilingualism.