The day the Nyan Cat crashed into the Twin Towers.
Contrary to popular belief, the destruction of the Twin Towers was caused by terrorists. This is incorrect.
This was merely Nyan Cat's orbit into Earth's atmosphere on it's quest to Nyan across the galaxy.
"Now class, today we will reflect briefly on 9/11, a day terrorists killed hundreds of Americans by the destruction of the Twin Towers."
*student raises hand*
"Don't you mean Nyan Eleven?"