WordNet (r) 3.0 (2006):

Flag Day
    n 1: commemorating the adoption of the United States flag in
         1777 [syn: Flag Day, June 14]

The Jargon File (version 4.4.7, 29 Dec 2003):

flag day
 n.

    A software change that is neither forward- nor backward-compatible, and
    which is costly to make and costly to reverse. ?Can we install that without
    causing a flag day for all users?? This term has nothing to do with the use
    of the word flag to mean a variable that has two values. It came into use
    when a change was made to the definition of the ASCII character set during
    the development of Multics. The change was scheduled for Flag Day (a U.S.
    holiday), June 14, 1966.

    The change altered the Multics definition of ASCII from the short-lived
    1965 version of the ASCII code to the 1967 version (in draft at the time);
    this moved code points for braces, vertical bar, and circumflex. See also 
    backward combatability. The Great Renaming was a flag day.

    [Most of the changes were made to files stored on CTSS, the system used
    to support Multics development before it became self-hosting.]

    [As it happens, the first installation of a commercially-produced computer,
    a Univac I, took place on Flag Day of 1951 ?ESR]


The Free On-line Dictionary of Computing (30 December 2018):

flag day

    A software change that is neither forward- nor
   backward-compatible, and which is costly to make and costly to
   reverse.  E.g. "Can we install that without causing a flag day
   for all users?"

   This term has nothing to do with the use of the word flag to
   mean a variable that has two values.  It came into use when a
   massive change was made to the Multics time-sharing system
   to convert from the old ASCII code to the new one; this was
   scheduled for Flag Day (a US holiday), June 14, 1966.

   See also backward combatability, lock-in.

   [Jargon File]

   (1998-01-15)