According to the United States Federal law known as the Flag Code:

• The American flag should not be flown after dark unless a light is fixed on it.
• The American flag should only be flown in fair weather (unless it was designed for inclement weather).
• When the flag is being lowered, it should never be allowed to touch the ground.
• The American flag should not be used as part of a costume or athletic uniform (flag patches may be used on the uniforms of military personnel, firemen, policemen and members of patriotic organizations).
• The American flag should be kept clean and free from rips. When it has become so worn it’s no longer fit to display, it should be burned in a dignified manner.

The Flag Code at usflag.org
Wikipedia – United States Flag Code