Well, it's on April 12th, as the nice HD wallpaper here shows!

Cosmonautics Day is a holiday celebrated in Russia (and some USSR countries) for the first manned flight in 4/12/1961 made by Yuri Gagarin (successful one), nowdays it's declared as the International Day of Human Space Flight.

So, just win 5 matches in 12th of April, and the achievement is yours! No big deal. :3
PS The biggest deal is to remember that you should do it.

Good luck!