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 short-lived 1965 version of the ASCII code to the
1967 version (in draft at the time); this was scheduled for Flag Day
(a U.S. holiday), June 14, 1966. See also backward combatability.
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