In 1999, UNESCO meeting emphasized the importance of the mother tongue and the need to preserve the heritage and cultural diversity related to the mother tongue. There are more than 7000 languages spoken all around the world. Except few hundred most of them are almost going to be extinct because of globalization.
In 1952, 21st February Bengali students of various Universities and Colleges rose up against the Pakistan's advocation of Urdu as the sole National language(In 1952, Bangladesh was part of Pakistan and it was called East Pakistan). They protested against this order and asked for inclusion of Bangla, their mother tongue as one of the two National Languages of the then Pakistan. When they were marching near the High Court, Dhaka few students were shot and killed by the police.
To honour those Bangladeshi martyrs, UNESCO decided to observe 21st February as the "International Mother Language Day" (IMLD). Since 2000, this day is observed as the IMLD all around the world to preserve linguistic and cultural diversity. Not only that, UNESCO announced 2008 as a International year of Languages.
Every year UNESCO observes this day with focus on some theme. In 2013 the theme was "Books for Mother Tongue Education" and 2014 it was "Local languages for global citizenship: Spotlight on Science". For 2015, the theme is "Inclusion in and through education: Language counts" in line with Nelson Mandela's quote "If you talk to a man in a language he understands, that goes to his head. If you talk to him in his language, that goes to his heart".
The above stamp issued in the year 2010 has not directly related to the Mother language Day but the background of the stamp shows four languages (including my Mother Language) Tamil, Telugu, Kannada and Malayalam, which are called Dravidian languages.