context: On February 7, 1987, dozens of political prisoners were freed in the first group release since Khrushchev's "thaw" in the mid-1950s. On May 6, 1987, Pamyat, a Russian nationalist group, held an unsanctioned demonstration in Moscow. The authorities did not break up the demonstration and even kept traffic out of the demonstrators' way while they marched to an impromptu meeting with Boris Yeltsin, head of the Moscow Communist Party and at the time one of Gorbachev's closest allies. On July 25, 1987, 300 Crimean Tatars staged a noisy demonstration near the Kremlin Wall for several hours, calling for the right to return to their homeland, from which they were deported in 1944; police and soldiers merely looked on.
question: ১৯৮৭ সালের আগে শেষ কবে বন্দীদের মুক্তি দেওয়া হয়েছিল?
answers: mid-1950s.