After Japan surrendered to the Allies on September 2, 1945, the Allied occupation government (aka SCAP or GHQ) helped to draft Japan’s new, post-war constitution, sometimes called the “Peace Constitution.” It was put into effect by will of the people on May 3, 1947, and Constitution Memorial Day was celebrated the following year. According to the Japanese government, Japan had already been a democracy since the Meiji Period, but this document clearly stated that the emperor was merely a symbol of state with no governmental powers, and so he remains to this day.
You can see a copy of the document online in The National Archives of Japan Digital Archives.