On April 26, 1986, Reactor #4 of the Lenin memorial powerplant experienced a critical meltdown which caused it to leak out highly radioactive clouds of smoke into the air.
Because of this, the area around Chernobyl was so irradiated that it was impossible for the environment to support a habitable life for humans, turning it into the contaminated wasteland we all know today. The huge clouds of smoke also threatened neighboring areas around it, such as Russia and Belarus, with the latter taking the heaviest hit from it.This event was then remembered by all the the worst nuclear powerplant accident in history.
However, we do not only remember the disaster on this day. Today we also pay tribute to the hundreds of men and women who risked their lives to control this disaster from spreading any further and many of them died from the radiation exposure, knowingly or unknowingly. Whatever the case however, all of them died as heroes protecting their people in a display of courage and selflessness, which all of us should take awe at.
You don't need to do anything fancy to pay tribute to them, just remind people of this day and what these heroes did to stop it.