Original: It was adopted on March 20, 1989, almost two years before the reestablishment of Lithuania's independence following the end of the Soviet Union. Before its readoption, the flag had been used from 1918 until 1940 when Lithuania was occupied and then annexed by the Soviet Union.
Simplified: The Flag of Lithuania that is used today was adopted by the Council of Lithuania in 1918 and re-adopted in 1989.