On 30 December 1922 the Soviet Union was founded. On 28 December 1922 a conference of plenipotentiary delegations from the Russian SFSR, the Transcaucasian SFSR, the Ukrainian SSR and the Byelorussian SSR approved the Treaty on the Creation of the USSR and the Declaration of the Creation of the USSR, forming the Union of Soviet Socialist Republics. These two documents were confirmed by the first Congress of Soviets of the USSR and signed by the heads of the delegations, Mikhail Kalinin, Mikhail Tskhakaya, Mikhail Frunze, Grigory Petrovsky, and Alexander Chervyakov, on 30 December 1922. The formal proclamation was made from the stage of the Bolshoi Theatre.
Source: ‘Soviet Union,’ Wikipedia