Source : WordNet®

Flag Day
     n : commemorating the adoption of the U.S. flag in 1777 [syn: {June
         14}]

Source : Free On-Line Dictionary of Computing

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)