Extract the answer to the question from the following context.
Question: What position had Gorbachev held until Christmas 1991?
Context: In a nationally televised speech early in the morning of December 25, 1991, Gorbachev resigned as president of the USSR – or, as he put it, "I hereby discontinue my activities at the post of President of the Union of Soviet Socialist Republics." He declared the office extinct, and all of its powers (such as control of the nuclear arsenal) were ceded to Yeltsin. A week earlier, Gorbachev had met with Yeltsin and accepted the fait accompli of the Soviet Union's dissolution. On the same day, the Supreme Soviet of the Russian SFSR adopted a statute to change Russia's legal name from "Russian Soviet Federative Socialist Republic" to "Russian Federation," showing that it was now a sovereign state.
President of the Union of Soviet Socialist Republics
This question is asking the position held by Gorbachev until Christmas 1991. The context tells us that he was president of the USSR, and specifically says that his resignation from this post ended all activities related to it.
Therefore, the answer is "President of the Union of Soviet Socialist Republics"..