At the partition of India in 1947, the Bengal province was divided according to the predominant religions of the inhabitants. The western part became part of India and the eastern part became a province of Pakistan known as East Bengal and later East Pakistan. However, there was economic, cultural and lingual friction between East and West Pakistan.

At the partition of India in 1947, the Bengal province was divided according to the predominant religions of the inhabitants. The western part became part of India and the eastern part became a province of Pakistan known as East Bengal and later East Pakistan. However, there was economic, cultural and lingual friction between East and West Pakistan.

These tensions were apparent in 1948 when Pakistan's government declared that Urdu was the sole national language. This sparked protests amongst the Bengali-speaking majority in East Pakistan. The government outlawed the protests but on February 21, 1952, students at the University of Dhaka and other activists organized a protest. Later that day, the police opened fire at the demonstrators and killed four students. These students' deaths in fighting for the right to use their mother language are now remembered on International Mother Language Day.
http://www.timeanddate.com/holidays/un/international-mother-language-day