21 February was declared to be the International Mother Language Day by UNESCO in 1999. It has been observed throughout the world since 21 February 1996. The declaration came up in tribute to the Language Movement done by the Bangladeshis (then the East Pakistanis).
When Pakistan was created in 1947, it had two geographically separate parts: East Pakistan (currently known as Bangladesh) and West Pakistan (currently known as Pakistan). The two parts were very different from each other in the sense of culture and language.
