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.
Source: Patriot Day | USPS News Link