The flag represents a living country and is itself considered a living thing. (U.S. Flag Code, Title 4, Section 8j)

It is the universal custom to display the flag only from sunrise to sunset on buildings and on stationary flagstaffs in the open. However, when a patriotic effect is desired, the flag may be displayed 24 hours a day if properly illuminated during the hours of darkness. (U.S. Flag Code, Title 4, Section 6a.)

The flag should not be displayed on days when the weather is inclement, except when an all-weather flag is displayed. (U.S. Flag Code, Title 4, Section 6c)

The flag should never touch anything beneath it, such as the ground, the floor, water or merchandise. (U.S. Flag Code, Title 4, Section 8c)

The flag, when it is in such condition that it is no longer a fitting emblem for display, should be destroyed in a dignified way, preferably by burning. (U.S. Flag Code, Title 4, Section 8k)

See U.S. Flag Code for further information on U.S. flag etiquette.