Peace Officers Memorial Day, which honors police officers who lost their lives in the line of duty, is Sunday, May 15.
Postal Service facilities that are open that day are required to fly the U.S. flag at half-staff.
Offices that are closed May 15 and fly the flag seven days a week should lower the flag to half-staff when the office closes Saturday, May 14, and return it to full staff when the office reopens Monday, May 16.
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: USPS News Link