The USSR installed a Soviet controlled puppet government in Hungary at the end of WW2. On the 23rd of October 1956, a student demonstration began in the capital city of Budapest. The demonstration attracted thousands of Hungarians to march in the streets. The delegation of students were demanding that their grievances be broadcast by radio when they were fired on by the State Security Police. One demonstrator was killed, and his flag draped body was carried above the crowd. This was the beginning of disorder and violence in the city.

The revolution quickly spread and the pro-Soviet government was overthrown. The new government quickly moved to establish free elections, and they made plans to leave the Warsaw Pact. At first the USSR was willing to negotiate the removal of their troops from Hungary, but then they changed their minds. On the 4th of November a large Soviet force entered Hungary to crush the revolt. The fighting continued until 10 November, and caused the death of 2500 Hungarians, and 700 Soviets.

After the breakup of the USSR, the 23rd of October was declared a national holiday in Hungary.

More Info: en.wikipedia.org