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Flag Day

/flāg dā/

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.