Probably the beginning of the Siege of Memel (Klaipeda). The Memelland had been part of East Prussia prior to WWI, after which it was placed under League of Nations control, and subsequently annexed by Lithuania, the territory itself being largely populated by Protestant and Catholic Lithuanians who lived as peasants on the lands surrounding Memel (Klaipeda). When the Soviet Union annexed Lithuania, the Germans were allowed to annex the Memelland.

In 1944, in the wake of the Baltic Offensives and Operation Bagration, Marshal Ivan Bagramyan's 1st Baltic Front entered the Memelland and surrounded the city, trapping a number of German units, including the GrossDeutschland Panzergrenadier Division, and in turn trapping Army Group North, which had escaped near encirclement in Latvia/Estonia via the Riga corridor, in the Courland Peninsula. Memel fell to the Red Army the following spring.

I was looking into this question and found the answer very difficult to answer. The major obstacle is defining the extent of German soil. Germany did a lot of conquering and annexation which caused a problem in defining what Germany saw as its borders and what other countries saw as Germany's borders.

I'm not sure if this answers your question, but the Soviets entered Warsaw on 17 Jan 1945. Warsaw had been near the Molotov-Ribbentrop border so could be considered part of Germany. The second could be the crossing of the Oder River by the 5th Shock Army at the end of January. Both were part of the Vistula-Oder Offensive.