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 CTSS 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. The actual change moved the code point for the ASCII newline character; this required that all of the CTSS source code, documentation, and device drivers be changed simultaneously. See also backward combatability. [Previous versions of this entry described this as a change in Multics, which was wrong. Evidently this confusion arose from the fact that the changes were made partly to facilitate Multics development --ESR] [As it happens, the first commercial installation of a computer, a Univac I, took place on Flag Day of 1951 --ESR]

--The Jargon File version 4.3.1, ed. ESR, this entry manually entered by rootbeer277.