Tiananmen Square
China has discouraged its citizens from talking about the Tiananmen Square Massacre. It's been erased from textbooks and is not covered in the school curriculum. Every year, the country tightens up internet censorship to make sure that no one talks about it around the anniversary.
The 1989 Tiananmen Square protests were led by students whose goals were to encourage democratic ideals in China. The protests and demonstrations began in April and were put to an end in June as China declared martial law and sent the military in to disassemble demonstrations. Hundreds, possibly thousands, of protestors and innocent bystanders were killed by the military.