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.^[1]

This usage originates from an obscure change in the Multics operating system's definition, which was scheduled for the U.S. holiday, Flag Day, on June 14, 1966.^[1]

Another flag day was January 1, 1983: when the ARPANET changed from NCP to the TCP/IP protocol suite.

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