4 U.S.C. § 6 : US Code - Section 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;
    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.