It's held on the 21st of February.
About 50 years ago, バングラデシュ was part of Pakistan. パキスタン barely gave a shit about us, and Pakistan's Government was going to make Urdu the state language, but we (the people of East Pakistan/present 日 Bangladesh)speak in Bengali. We always have, always will. That new law angered us, and we started having strikes and riots. This made the Pakistani Government angry, and they sent down officials to shoot the protesters. They even shot students in Dhaka University! In the end, we got what we wanted. For years, people of East パキスタン would go to the Shaheed Minar and place お花 at the steps to honour those who died for their Mother Tongue. A few years later, UNESCO declared the 21st of February to be International Mother Language Day, for we are the only country where people had to DIE for their language!
"Amar Bhaier Rokte Rangano Ekushe February. Ami ki bhulite pari?" Is a song that was written for this day. This says "Intimidate the blood of my brother on the 21st of February. How can I forget this?" link