4 U.S.C. § 6 - U.S. Code - Unannotated Title 4. Flag and Seal, Seat of Government, and the States § 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.

(d) The flag should be displayed on all days, especially on New Year's Day, January 1; Inauguration Day, January 20; Martin Luther King Jr.'s birthday, third Monday in January; Lincoln's Birthday, February 12; Washington's Birthday, third Monday in February; National Vietnam War Veterans Day, March 29; Easter Sunday (variable); Mother's Day, second Sunday in May; Armed Forces Day, third Saturday in May; Memorial Day (half-staff until noon), the last Monday in May; Flag Day, June 14; Father's Day, third Sunday in June; Independence Day, July 4; National Korean War Veterans Armistice Day, July 27; Labor Day, first Monday in September; Constitution Day, September 17; Columbus Day, second Monday in October; Navy Day, October 27; Veterans Day, November 11; Thanksgiving Day, fourth Thursday in November; Christmas Day, December 25; and such other days as may be proclaimed by the President of the United States; the birthdays of States (date of admission); and on State holidays.

(e) The flag should be displayed daily on or near the main administration building of every public institution.

(f) The flag should be displayed in or near every polling place on election days.

(g) The flag should be displayed during school days in or near every schoolhouse.