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}.