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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 on Flag Day (a U.S. holiday), June 14, 1966. This change bundled together at least two different conversions. One was from the short-lived 1965 version of the ASCII code to the 1967 version (in draft at the time); this the moved code points for braces, vertical bar, and circumflex. Another was a rewrite of the CTSS filesystem. See also backward combatability.
[Previous versions of this entry described this as a change in Multics, which was wrong, and also got the code points changed in the character set wrong. Evidently some of this confusion arose from the fact that the changes were made partly to facilitate Multics development --ESR]
[As it happens, the first installation of a commercially-produced computer, a Univac I, took place on Flag Day of 1951 --ESR]
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