From USPS News Link:
As a mark of respect for the victims of the attack in Boston, President Obama has ordered that the flag of the U.S. be flown at half-staff at all public buildings and grounds, at all military posts and naval stations and on all naval vessels of the Federal Government in Washington, DC, and throughout the U.S. until sunset, April 20, 2013.
Under this order, postal facilities are required to fly the flag at half-staff. When flown at this position, 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.