That would be Khrushchev's secret speech in 1956. Nikita Khrushchev succeed Stalin as General secretary after Stalin's death and a period of power struggle which resulted in the death of Stalin's former secret police chief Beria. Khrushchev, who was once a member of Stalin's inner circle, made a somewhat surprising speech during the 20th party congress, denouncing Stalin's cult of personality and among other things:

Compliance in the Great Purge of the 1930s and the execution of hundreds of thousands of innocent people, including high ranking Communist party officials who were part of the October revolution.

Deviation and violation of the principles of Marx-Leninism and norms of Communist party.

Deportation of ethnic groups (i.e the Chechens)

Orchestration of the anti-Semitic "doctor's plot"

And a whole bunch of other things.

This came as a great shock to many Communist party members and Soviet citizens because of the intensity of propaganda portraying Stalin as a titan and hero of the Soviet Union and the international Communist movement. The speech marked a period of de-Stalinization in Soviet history known as "The Khrushchev Thaw".

Excellent responses. The BBC Radio 4 did several excellent programs about this, but good luck finding them. Check out "The Whisperers: Private Life in Stalin's Russia" by Orlando Figes.

From a Soviet defector, I heard an apocryphal story about one of Khrushchev's later (many) speeches blaming everything on Stalin. He was speaking to huge audience at a Soviet military college. As Khrushchev droned on about all of the Boss' crimes, on officer in the back shouted out, "Well, why didn't you do something?" Khrushchev demanded "WHO SAID THAT?!?" to be met with a terrified silence. He smiled coldly and said, "Now you know why."