4 U.S. Code 1 § 6 - Time and occasions for display

(a) 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. (b) The flag should be hoisted briskly and lowered ceremoniously. (c) The flag should not be displayed on days when the weather is inclement, except when an all weather flag is displayed.

https://www.marines.mil/Portals/1/Publications/MCO%20P10520.3B.pdf

4 U.S. Code 1 § 6 - Time and occasions for display

(a) 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. (b) The flag should be hoisted briskly and lowered ceremoniously. (c) The flag should not be displayed on days when the weather is inclement, except when an all weather flag is displayed.