Sept. 11 is Patriot Day, when Americans pause to remember the 9/11 terrorist attacks.
All postal facilities must fly the American flag at half-staff on this day.
Patriot Day, also known as the National Day of Service and Remembrance, will include a moment of silence at 8:46 a.m. EDT, the time the first plane struck the World Trade Center on Sept. 11, 2001.
To fly the flag at half-staff, hoist the flag to the peak for an instant and then lower it to the half-staff position. The flag should be raised to the peak again before it’s lowered for the day.
The World Trade Center’s Twin Towers before terrorist attack on September 11, 2001
Source: Patriot Day | USPS News Link