On 21 February 1952 Bangladeshi brave child gave their blood for mother tongue Bangla.Pakistan declared that "Urdu" will be national language of Pakistan but East pakistan oppose it coz their language was Bangla.
In history only Bangladeshi people gave their blood,no other nation did that.That's why UNESCO declared 21 February as "International Mother Language Day".In Bangladesh it's known as "Shahid Dibosh".