In September 2011, hackers installed 153 (= 9 × 17) custom-made wirelessly-controlled color-changing high-power LED lights into every window above the first floor of the tall MIT Green Building. They displayed a waving American flag throughout the evening of September 11, 2011 in remembrance of the September 11 attacks of 2001. For a short time in the early morning of September 12, the lights displayed a Tetris game, thus realizing a long-standing hack proposal, the "Holy Grail" of hacks. The display hardware had occasional glitches, and was removed as of September 13. The hardware and software designs were further developed and refined for better reliability. On April 20, 2012, MIT hackers successfully turned the Green Building into a huge playable Tetris game, operated from a wireless control podium at a comfortable viewing distance in front of the building. Visitors to Campus Preview Weekend (a gathering for admitted prospective freshman students) were invited to play the game on the colossal display grid, which was claimed to be the second-largest full-color video display in the US.