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Flag day, as used in system administration, is a change which requires a complete restart or conversion of a sizable body of software or data. The change is large and expensive, and—in the event it doesn't work—reversing the change is similarly difficult and expensive.
This usage originates from a change in the Multics operating system's definition of ASCII, which was scheduled for the U.S. holiday, Flag Day, on June 14, 1966.
Another flag day was January 1, 1983, when the ARPANET changed from NCP to the TCP/IP protocol suite.