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flag day

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 massive change was made to the Multics timesharing
   system to convert from the old ASCII code to the new one; this was
   scheduled for Flag Day (a U.S. holiday), June 14, 1966.  See also
   backward combatability.