Flag day
<jargon> A software change that is neither forward- nor backward-compatible, and which is costly to make and costly to reverse.
E.g. "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 time-sharing system to convert from the old
ASCII code to the new one; this was scheduled for Flag Day (a US holiday), June 14, 1966.
See also
backward combatability,
lock-in.
[
Jargon File]
