Which event in 2011 was cause for celebration in the united states?

Answer: death of Osama Bin Laden.

Explanation:

The event that was the cause of Celebration in 2011 was the death of Osama Bin Laden, at the hands of the US forces. The Afghan terrorist leader of the Al Qaeda network was finally killed. This event brought together thousands of people, most of them victims of the 2001 attack on the Twin Towers.

The crowd gathered at ground zero, where security measures were taken for precautionary reasons, in order to avoid a possible attack again.