Why was October 21st, 2015 known as Back to the Future Day?

Why was October 21st, 2015 known as Back to the Future Day? Because in 1989, a sequel was made to the blockbuster movie Back to the Future, somewhat predictably named Back to the Future II. In the first film. Marty McFly had travelled back in time to 1955 – in the sequel he went forward in time to – you guessed – October 21st, 2015. In 1989, I imagine that 2015 seemed to be in the distant future — yet here we...