1917-1920
After the 1917 February Revolution in Russia, the Belarusian language became an important factor in the political activities in the Belarusian lands (see also: Central Council of Belarusian Organisations, Great Belarusian Council, I All-Belarusian Congress, Belnatskom). In the Belarusian People's Republic, Belarusian was used as its only official language (decreed by Belarusian People's Secretariat, 1918-04-28). Subsequently, in the Belarusian SSR, Belarusian was decreed to be one of the four (Belarusian, Polish, Russian, Yiddish) official languages (decreed by Central Executive Committee of BSSR, February 1921).
Read more about this topic: Belarusian Language, History